Summary: Most smartphone owners upgrade every 2 years (if on contract), or when the battery life or performance becomes unacceptable (~ 2-3 years), or when the phone breaks and requires expensive repair that isn't considered worthwhile.

Yet, there is no inherent reason why a well looked after smartphone can't be used for 4 to 6 years if the battery (genuine replacement cost around $25-40) is replaced every 2 years and the operating system is not updated beyond a version the phone hardware can handle with adequate performance.

I bought my first smartphone (the Google Nexus 4 for $420) in March 2013 and in May 2016 replaced the battery myself with a supposedly genuine ("OEM") replacement that cost $16 on eBay (including tools). But it turns out ultra cheap eBay batteries are typically counterfeit or low quality and much more prone to issues like swelling, overcharging and thermal runaway. The replacement battery swelled and started playing up after 7 months, also cracking the case on my phone and prematurely ending its life.

My next Android phone will cost around $220 and I will only use a genuine OEM replacement battery (~$40). I'll probably use 2 protective cases/bumpers in that time (~$40). I expect to get 6 years of use from the phone which means the hardware cost will be ~$50/year. In comparison, a typical Australian can easily spend on average $400/yr on smartphone hardware. After 20 years of smartphone ownership this $350/yr saving will amount to $11,573 (5% return).

Details:

1. Quick guide to extending the life of your smartphone at the lowest cost

- Buy a durable smartphone from a reliable manufacturer running on an operating system that will definitely be around in 5 years time (currently this means Apple iOS and Android only).

- Do some research beforehand to ensure upgradeable or shorter-life parts can be readily and affordably upgraded.

- Protect the phone from accidental damage (case, screen protector) and preventable malfunctions. Also minimise the chances of losing it permanently or having it stolen - including using free locate/lock device apps like: Android Device Manager

- Avoid unnecessary major upgrades of the operating system and research performance impacts before you do upgrades. E.g. If you bought an iPhone with iOS v4, you may need to quit upgrades beyond v5 or v6. Android auto-upgrades are harder to prevent but tend to support older phones longer. If necessary, rollback versions where possible.

- Follow guides to improve performance including turning it off/on regularly, doing a full reset, closing open apps, using a memory and background process clearing app, deleting unnecessary apps and data, restoring to a clean backup or factory settings. E.g. See: Macworld - How to speed up a slow iPhone

- Preserve the battery life by following recommended practices. Smartphones now use lithion ion batteries that shouldn't be completely discharged. It's best not to discharge below 20% (ideally 40%) and to recharge often but preferably to 80%. Lithium batteries are best kept charged between 40-80%.  Don't leave the phone plugged in on full charge. Every few months a full discharge and recharge may be useful to recalibrate the battery.

See:
> How-To Geek - Debunking Battery Life Myths for Mobile Phones, Tablets, and Laptops
> Android Pit: How to calibrate the battery on your Android phone or tablet

- Also implement guidance on resolving issues with fast battery drain. E.g. For Google Nexus phones see: My device's battery drains too fast

- When the battery life becomes poor buy a new, genuine OEM battery  and use a DIY guide to replace it yourself (or you can pay ~$50 to have it done for you). You can check battery health and capacity using free apps. E.g. Battery Monitor Widget (Android).
See: How-To Geek - How Do You Know When It’s Time to Replace Your Battery?

- There are various ways to extend smartphone performance as it gets older: remove unused apps, run cleanup apps, keep at least 15% of storage capacity free, turn it off and on occasionally. You can even do a factory reset to start from a clean slate.
See: Life Hack - 10 Ways to Extend the Life of Your Smartphone

- Use DIY fixit websites like Ifixit to find out about how to fix any other repairs that may be needed.

My Google Nexus 4 was bought in March 2013 for $420 all up.
+ $10 for a generic bumper on eBay to protect the screen
+ $16 for an OEM battery and tools on eBay to replace the battery in May 2016


2. Smartphones are deliberately designed to make repair and battery replacement difficult and expensive so that consumers purchase new ones earlier than they otherwise would

This occurs via:

- Making batteries and other upgradeable parts difficult to access and replace oneself

- Voiding warranties and support agreements if devices are opened or repair/replacement is done in unauthorised ways

- Implementing proprietary parts that can only be obtained via expensive, official channels

- Pushing out software updates that adversely impact the performance of older phones but cannot be rolled back

See:
> Gizmodo: Your New Galaxy S6 Will Have a Built-In Expiration Date
> Does Apple Ruin Your iPhone on Purpose? The Conspiracy, Explained

3. Like many products smartphones are subject to planned obsolescence but this can be avoided

At first, I thought it was my imagination. Around the time the iPhone 5S and 5C were released, in September, I noticed that my sad old iPhone 4 was becoming a lot more sluggish. The battery was starting to run down much faster, too. But the same thing seemed to be happening to a lot of people who, like me, swear by their Apple products. When I called tech analysts, they said that the new operating system (iOS 7) being pushed out to existing users was making older models unbearably slow. Apple phone batteries, which have a finite number of charges in them to begin with, were drained by the new software. So I could pay Apple $79 to replace the battery, or perhaps spend 20 bucks more for an iPhone 5C. It seemed like Apple was sending me a not-so-subtle message to upgrade.
NY Times Economix: Cracking the Apple Trap

See:
> NY Times Economix: Planned Obsolescence, as Myth or Reality

4. As of 2016, new must-have innovations in smartphones are very rare and there is little need to upgrade every few years

Smartphones have matured and reached a level of quality and capability that satisfies most people’s needs.... To be clear, I’m not saying that there isn’t any innovation going on in smartphones — there clearly is — but once people get a five-inch or larger HD display, a good quality camera, lots of storage, speedy network connections and access to millions of applications and services, most people think their phone is “good enough.”
Recode: The Smartphone Lifetime Challenge

The main reasons I have a smartphone rather than dumbphone are:

- WiFi access
- Google Maps
- WhatsApp (virtually free messaging/chat rather than SMS)
- Gmail and Google Calendar/reminder access
- Facebook access
- Web browser access for mobile-friendly sites (news, blogs, sports betting, etc)
- Sending photos easily to my PC/email/others
- Using Pocket to read articles offline
- Using Kindle to read books offline
- Weather apps (especially checking rain radar)
- Cycling apps (Strava, RideWithGPS)
- Sports apps (Footy Live, SportsFan)
- Using Player FM to listen to podcasts offline
- Viewing docs on my phone
- Online bank/investment apps
- Google Hangouts/Skype video chat
- P2P apps (Spinlistet bike renting)
- Location-based apps (Snap Send Solve)
- Transit apps (tramTracker, Train Trapper)
- VPN access to get around geo-restrictions (Tunnel Bear)
- Video playing apps

None of these things require a recent smartphone. The apps they depend on are updated automatically and continue to work on the Nexus 4 and other cheaper Android smartphones with no significant issues.

Since 2015, advertising for smartphone upgrades has focused primarily on inessential features like:

- Gradually improving cameras and screens
- Voice commands
- Waterproof ability
- Fingerprint security
- Photospheres, "Live Photos"

These simply aren't things many people really need.


Further Info:

Cult of Mac
> Macbook Air Teardown Reveals Proprietary Engineering
> Is Apple Guilty of Planned Obsolescence?

Gotta Be Mobile
> How to Downgrade Android 5.0 to Android 4.4

NY Times
> Choosing to Skip the Upgrade and Care for the Gadget You’ve Got

Imaging Resource
> Counterfeit batteries: Clever fakes, uncertain risks

How-To Geek
> What to Do When Your Phone or Laptop Has a Swollen Battery
> Danger: Why You Shouldn’t Buy Cheap Third-Party Batteries For Laptops or Smartphones