I expect the support to be later removed even for apps not in the app store. As hinted by Dominus (with the dual bit binaries), this will imply a lot of disk space reserved by the OS can be reduced, by removing 32-bit executable code (including shared libraries) and other related code pieces.
This can also reduce memory usage in certain circumstances. More shared libraries' code currently has to be loaded when separate 32-bit and 64-bit exes are being run concurrently, even if they depend on 32-bit and 64-bit builds of the exact same shared libraries.
What's less clear, is if they can remove the support from the hardware. I suppose the answer is "No", as long as Intel and x86-64 are used. The situation is quite different with Apple's ARM-based processors, though. (x86-64 is essentially in extension of i386, whle ARM's AArch64/A64 is more-or-less separate from A32.)