What do you mean by 'ROM' or 'burnt'? Android is normally on flash storage. Even if a device does not get updates anymore, you could still replace it if the boot loader is unlocked (e.g. Cyanogenmod).

Of course, you know this, so I can only assume that you are using such words for dramatic effect ;).

Which normal users are unlocking their devices?

It doesn't matter if it is flash, eprom, rom, whatever.

The practical result is that most devices die with the Android version they were bought with.

Normal users do not care about firmware versions or cpu optimizations.

An app either runs, or not. Dalvik allows the apps to run, without the user having to know about cpu architecture in their phone or whatever.

But I as developer care to deliver the best experience in terms of performance from Android 2.3 all the way up Android 5.0.