Not your point, but Linux compatibility is Valve protecting themselves from the existential risk that is a capricious Microsoft. At one point, it seemed Microsoft was trying to make the Microsoft Store be the mechanism for distributing all software. Linux being viable for gaming is/was their safety net to avoid being locked out from the ecosystem.

Are there even close to 2% of Debian users using a DEC Alpha?

popcon.debian.org reports 3 alpha installations and 261750 amd64 installations. Assuming comparable opt-in rates there are less than 0.002% of the users using alpha.

The other mentioned architectures hppa, m68k and sh4 are at a similar level.

Valve isn't a good example. They have strong Linux support so they can sell Steamdecks without licensing with Microsoft. Without their work on Proton, Steam effectively lives or dies by the will of Microsoft.