I'm sorry but entirely missing is IMHO "the real elephant" in the room...

that being that Steam has outperformed Microsofts own stores on the PC platforms.
(be it generically Microsoft or Xbox fronted)

Microsoft DOES have the resources to purchase Valve however its owner needs to be willing to sell.

However the assumption here is that any 'leveraging' of Steams catalog is merely Microsofts attempt to 'shoehorn' itself into Valves revenue stream by getting 'kick backs' from selling access to Steam games with Microsofts own store front.