[sherincall]

> And the only cost is that old binaries (not old code, old binaries) will no longer run without being rebuilt.

Not always true. There's plenty of 32bit code that can't trivially be ported over to 64bit, even if you have the full source for all the dependencies. e.g. maybe the file formats have padding or pointer width assumptions - now you need a compatibility shim whenever loading/saving the files. Maybe the code has implicit padding assumptions, which are not documented anywhere - now you'll get occasional random functionality bugs that are a huge effort to catch and debug.

It is not always a matter of flipping a compile-time switch for the developers, even if they are still around, and have all the source, and all the libraries and whatnot.