[iSnow]

If anything fails in your ubuntu install, you'll go and find the answers on Ask Ubuntu or so, whereas people expect their smartphones to "just work" and work consistently without changing driver settings.

That creates test and QA work on Apple's part. They also have a pretty good breakdown how many devices of which build type are still out in the wild - and at some point, it simply becomes uneconomical to still support a given device version.

You aren't wrong to point out that incentives are wrong since we don't pay for the Android/iOS version, so the manufacturer makes more money retiring support early, but IMO Apple is doing a pretty good job here, esp. considering how much smartphones have evolved during that time.