Yes—and my concern is that old binaries will suddenly stop working on newer hardware. This has happened before! The whole reason the top bits are sign-extended

Yes—and my concern is that old binaries will suddenly stop working on newer hardware. This has happened before! The whole reason the top bits are sign-extended in the first place is to discourage these kinds of shenanigans. I remember the pain of switching 24-bit to 32-bit on M68K Macs.