More than that, by diversifying Valve is getting insurance from Microsoft bad mood in 2 ways
If the experience gets shitty because Microsoft manipulate Valve and games into making gamers getting bad experience, they can say "works just fine in my steam deck"
And if they try to do the "30% subscription/marketplace tax" that iOS and Android pull on devs they can go rogue and say it's cheaper on steam deck/Linux which would also cause a big kerfuffle