TBH I don't think they'd need to buy either of those companies (and for various reasons, couldn't buy Valve without probably having it ruled as anti-competitive), just a partnership.
While their latest investments in Epic probably aren't specifically tied to EGS, I wouldn't be surprised if that is at least a factor into the investments. Epic already only take 12% of the cut; maybe they reduce that to 10% with Sony's games, and Sony might look to publish certain 3P games on PC or co-publish PC versions of certain 3P games.
Or possibly Epic waives their cut for Sony's games altogether but they work on integrating PS+ features and compatibility into EGS, and tie subscription perks into EGS to match PS+ ones (current and forthcoming). That would also incentivize more 3P publishers to bring their releases to EGS as the favored storefront. I wouldn't expect them or Sony to stop bringing their games to Steam, just that there could be benefits like the aforementioned and maybe some others (like a 10% discount) for buying them on EGS vs. Steam.
It's an interesting possibility but it's probably a decent ways out before seeing if there's any movement towards it.
I agree, they don't need to buy these companies (even if would be good acquisitions) and whatever Sony does or acquires they'll continue publishing the Sony PC ports in Steam for many years because they have a big majority of the PC market.
Regarding Epic, the biggest asset they have is Unreal Engine. It's becoming a monopoly for AA/AAA games, and is growing in other markets like shooting movies or tv shows, tv ads, or in other areas like architecture. If Sony would want to buy Epic and keep them working independently multiplatform and selling UE to everybody, they could have issues with regulators due to game engines monopoly.
At some point, Sony is going to unleash a shit load of PC ports. Right now there's only a handful that sporadically come out every 6 or 9 months or whatever. And the Uncharted 4 port is coming out in 2022 too at some point.
There is no way Sony bought Nixxes, hiring PC Director roles and doing PPT slides promoting PC gaming if their plan is just to continue doing 3 year old ports every 9 months kind of thing.
If Sony's future plan was to do that, they wouldnt even need Nixxes or all these new PC people. They'd just keep porting it how they do now with existing teams.
I think Sony will increase their amount of PC ports to 4, maximum 6 per year. They require marketing, and they also market their PS games, services and hardware. To go beyond that very likely would saturate their marketing schedule and the budget of the PC players. Like all PS Studios Nixxes is growing to be able to work in more projects. Until then I think they'll continue hiring people like Iron Galaxy to help them with the ports.
We also have to remember that in the console side Sony is acquiring studios, growing the existing internal ones they have and if Jim Ryan said PS5 will be their console with more exclusives I think he meant they'll have more 3rd party exclusives than ever, because always a big majority of their exclusives have been 3rd party. So they may reach a point where they could saturate not only their marketing channels and PC players, but also console players.
So maybe 4 PC ports per year (each one of them could be a collection/bundle as the upcoming Uncharted one) is the sweet spot. I think TLOU 1+2 Director's Cut (TLOU remake + TLOU2 PS5), Spider-Man + Spider-Man Miles Morales, Uncharted Nathan Drake Collection and Bloodborne Director's Cut (remastered+new content) and Ghost of Tsushima are very good candidates from late 2022 to early 2024. Add there the upcoming Bungie new IP which will be multiplatform day one everywhere.