Considering the state of antitrust regulation these days, as proven by the ABK/Bethesda mergers there is a lot of room MS has to work with if they decide to acquire Valve and a fart is sent their way by antitrust bodies. Much easier since it's a private company as well in terms of reaching a deal and avoiding causing a furor from competitors (who will lobby for action). Nobody depends on Steam for their dough - not Apple, not Amazon, not even Sony. Nobody depends on Valve content either cause Valve dropped the software game a long time ago. Valve is picture made for a Microsoft acquisition. They'll definitely cite Apple's and Google's Android's storefronts (Apple store, Google Play Store), bring the Epic vs. Apple ruling to the forefront as a talking point.... get the right judges, examine the make up of the regulatory bodies (who's buyable, who's not, what the angle will be - position resources accordingly in anticipation) and work from there. I expect them to at least be infinitely more prepared to subvert and corrupt the process due to experiences with the ABK deal. They already got their plant in the EU. Although the EU commission is not a roadblock in any way shape or form clearly, regardless of.
I guess we're all free to project what we think will, could happen theoretically. I'm of the opinion, in the current environment, a MS/Valve merger will face much less hurdles, specially if the ABK deal fails and thus it does not become a weapon antitrust regulators can use to bash MS's head in. Of course there is going to be a lull, as a strategy is deviced - acquisitions and mergers all serve a strategy - they're not just "wants". We're also just talking PC gaming here, a software subset class on Windows. The only thing you're acquiring is a software distribution operation (a middleman), not content that can be taken away strategically from competitors. Half Life/Counter Strike ain't key to anybody.