“So what you actually may get is runtime breakage, potentially going as far as to create security issues.”
A 32-bit signed time_t breaks in 2038, and widening it changes the layout of every struct that carries a timestamp. Mix one old library with one new program and they read the wrong fields. Gentoo has to rebuild the whole system in place, so a half-finished upgrade is a live risk. Prebuilt proprietary 32-bit binaries get no fix at all beyond faking the clock.