Backwards compatibility at any cost?

IMHO the only incompatibility is that for addresses smaller than 16 bytes the kernel will no longer zero out the padding when it copies an address to userspace / require the padding to even exist when it reads an address.

Everything else should literally stay the same. I have no idea how many programs go splat because their address padding is no longer overwritten but if the number is nonzero then a compatibility mode can be added to the place where the address is copied to userspace. Not spread across the whole of the kernel.