I've never submitted an app, but do they submit source code? If not, How can they recompile if only binaries are submitted?
I'm a programmer and have written both compilers and cross compilers. You don't need the source code. If you have the final program you can run it through a cross compiler, a just-in-time compiler or just emulate to run it on different hardware. That list is in order of efficiency and preference. This is not a hard task. Apple has done it before.
I don't think this is that easy of a problem to solve, and why should the onus be on Apple to update old stuff? I don't understand that logic at all.
It is an easy problem. The logic is also quite simple. The reason Apple should be interested in doing this is it adds value for their customers because the software you use today will run tomorrow and because it maximizes the Apple application ecosystem which Apple likes to crow about in their marketing materials, ads, etc.