It will never happen, but my dream is for the Asahi devs, Valve, and Apple to all get together to build out a cross-platform Proton to emulate and play games built for Windows on both x86 and ARM hardware running Linux.
A Steam Deck with the performance and power efficiency of an M-series ARM chip and the entire library of games that run on Proton is just...dreamy.
The end game for Valve isn't Steam Deck 2 or 3 (which is statistically impossible for Valve to produce), but for Steam to be on everything.
However, it is Microsoft more than anyone else that has decided to stop supporting those operating systems. Windows XP does not have support for any modern version of TLS (only TLS 1.0). There's no good way to support a browser-based app like Steam on a platform that cannot natively provide a secure connection to a modern web server.
There is not such a hard reason to drop Windows 7 support (again, except that Microsoft no longer supports it) but there are security-relevant APIs that are only available starting in Windows 10 which means special patches would have to be maintained just for Steam on Windows 7 to continue working securely.