“version numbers never change” (which is what stable means in Debian Stable).

My interpretation of stable isn’t just versions not changing, only that the bugs are known and newer ones aren’t easily introduced, i.e. the state of the system is know. While rolling releases are fantastic for end users and to obtain the latest software, sometimes a particular bug or change will modify a user’s workflow.

Ah. I understand now. I can see most distros dropping support for 32-bit libraries if this happens, my guess would be distro derivatives mantainers taking this burden instead. It’s not gonna be pretty for old or abandoned 32-bit linux builds.

Steam could build for 64bit

Steam is doing that, though rather piecemeal. Both their client and runtime are moving towards 64-bit.

but it won’t help the gigantic amount of games that are 32bit also.

Doesn’t WINE’s WOW64 implementation solve this? Albeit with a non perfect compatibility.

because macos bo longer ships with the 32bit libraries needed.

I have no idea what the MacOS software landscape looks like, but if it uses WINE it might just be its silver bullet? If Steam does move towards 64-bit only, they would want to keep it compatible with Linux, due to their SteamOS ecosystem, and in turn MacOS users could also reap the benefits.

to drop x86 32bit things will start to fall apart quickly

Why dropping x86 32-bit is an issue?