Vinyl got left behind by cassettes (for some people at least).
Everything is 'left behind' by whatever replaces it.
But when you own a smartphone, you don't necessarily know if it'll play apps. When I bought my phone, it was the current android release and it had everything I wanted in a phone. Now, it still has everything I want in a phone, but they aren't updating the OS. I could mod it to support android 4.x, but i'll lose some of the functionality of the device. This is not so uncommon, and it's not likely to stop happening any time soon. The solution I tend to get is that I just have to deal with it or buy a new phone. That seems silly to me, but perhaps I seem silly to you.
I think it's about abstractions and where you draw the line of 'different thing' - I see 'If you own an iPhone 4s it will play the apps for it until it dies' and 'If you own an Android phone with 4.x on it, it will play the apps for it until it dies' as being equivalent with 'If you own a cd player it will continue to play CDs'.
You're blurring the lines between CDs, Vinyl, Cassette, Hi-CD, SACD, DVD, HD-DVD, and BluRay audio as being all entirely unique 'universes', while smooshing all of the smartphone world into one 'universe'.
I see the difference between an iPhone 4s and an iPhone 5 as more like CD->DVD: similar, but distinctly different. Sure, lots of content works on both, but then, lots of content doesn't.
It's just now, with software-based (primarily) differences, that people even THINK about backwards compatibility/etc. as a feature to offer. CD players didn't offer backwards compat. with vinyl albums, for example (although, this argument might be flawed in that most future optical-disc drives did compat. back to CD).
With software, we CAN provide backwards compatibility, and you're argument (if I do understand properly) is that because we can, we probably should. I am torn by that, and can see strong arguments on both sides of the fence.
An iOS or Android app, on the other hand, is architecture-or-platform-specific code, depending on a particular OS-sized API and all its UI libraries, all of which are expected to become obsolete and unusable in a relatively short time.