In other words, you're simply making an _assumption_ that Valve will instantly drop support at Microsoft's EOL date.
I wish Valve actually spoke up against this EOL date, yet what they've done is just follow that date so far.
In the meantime, instead of ditching Steam, ditch Windows. Steam works amazing on Linux, and with Proton, the vast majority of Windows games Just Work(TM). You don't even need SteamOS (although it helps).