This doesn’t make any sense. The reason Valve hasn’t been acquired is because it’s privately owned and not up for sale, not because it doesn’t have “enough profit”. In fact it’s extremely profitable, for all we know.
Sure, another company could come along and build a competitor. It’s happened already multiple times, and Steam is doing just fine despite some major titles these days being exclusive to other platforms. Unless Steam drops the ball on something big time, it’s unlikely that people will move to another platform en masse, especially one that is less focussed on consumer interests. No-one can just come in and “take capital away” from Steam, whatever that means, by building a competitor that sells advertising space and “monetizes user data” — they need users first.
… And then there’s the fact that Steam is already “selling advertiser space” today. Games don’t just get featured on their storefront because Gabe likes them. They make deals with publishers for this.