I think comparing the PC market to Windows Phone is a bit different. Windows Phone required you to buy a specific piece of hardware. Which gaming platform you use (outside of the consoles) simply requires a PC, Mac, or Linux box. You can clearly mix and match using cloud gaming, or a locally installed game, depending on what you want to do, which is something you cant do when your locked into a specific piece of hardware.
Anyway you asked about Steam and Epic, so I'll try and answer.
The PC gaming market is basically split in 3 ways to get your games:
1. What I'll call the Primes - Steam, Epic, EA's Origins, Ubisoft's Uplay, and to some extent Gog
2. Key Resellers
3. Cloud Gaming/Streaming/Subscriptions
The Primes offer effectively an online library (and naturally a store attached to that).
Steam for example is pretty much the de facto PC gaming library. It offers tens of thousands of games in its store, which when you buy appear in your library. It offers easy ways to sort the library, as well as things which interest a lot of gamers, such as reviews, achievements, networking (chat with friends or groups (in game and out), screenshots, even live streaming), mod workshop, guides, etc. It is where most people keep their games.
Epic is Steam's biggest competitor. It also has a library and a store with thousands of games. It's not as big or as well stocked with features as Steam, but it's won a lot of customers through things like exclusivity deals, free games (there's a new free game available every couple of weeks), and taking a lower percentage from sales than Steam. It lacks a lot of the features of Steam like mods, most networking, etc. But yeah it still has a large userbase.
EA's Origin, and Uplay, both exclusively sell their own published games. For a while both of them stopped selling their games anywhere else, but both are now back selling their games on Steam and Epic, although often opening a game through Steam, opens up (for example) Uplay, before it launches the game. A little annoying, but thats where it's at.
Gog sits in a space between The Primes and the Resellers. They sell directly old Games (GOG stands for Good Old Games), and DRM free versions of games. They also sell Steam keys (often you get both a DRM free version and a steam key). This means you can have both a downloadable installer, and also have the game in your steam library for download whenever you want. Gog also has it's own library Gog Galaxy, which actually can be linked to Steam, Epic, and Uplay, to let you see (and download) all the games you have in your other libraries. A very handy feature.
The resellers, both offical and otherwise, sell keys which let you add games to your Primes library. The vast majority are Steam key resellers, although some sell Epic keys, and often any game from Ubisoft will actually be a key for Uplay. Some official Resellers are Fanatical, Humble, Green Man Gaming, Game Billet, and Instant Gaming.
Subscription/Cloud Gaming is basically things like the Xbox game pass, Playstation gaming pass, ubiconnect,. etc. Things like Stadia used to be in this category, but have effectively shut down. Basically, for these you pay a monthly subscription, and get access to a rotating selection of games.
As a general rule, all of these services on PC are platform agnostic, as in they exist on PC, Mac, and Linux. Epic is I think very limited on Linux, and I dont think Origin exists at all on Linux. But steam has done a lot to try and get more games available there. However, a lot of that comes down to the game publisher, and whether they want to make it easier to port or not.
I could then go into things like the Steam Deck which is a bit of a game changer in the hand held market (previously dominated by the Nintendo Switch), where games you own on Steam already are free to played on the Deck as well (no need to buy again, although assuming it runs on the hardware of course). But that's a whole other discussion.
To bring this back to the Microsoft Discussion - Xbox Game Pass already is the dominant player in subscription. It isnt the dominant player overall (that would be Steam), BUT it's easy to see how bringing the CoD's or other massive games from Activision Blizzard to Game Pass with exclusivity periods or making it the "optimum" experience would drive massive uptake of the subscriptions, and that would drive a massive cash cow into Microsoft's hands at the expense of everyone else. The thing with subscription services is that you dont actually own any of the games you're playing, so once it's removed from rotation that's it, it's gone. If you want to keep playing it, you have to go buy it somewhere else. Admittedly, legally speaking it's thankfully not been tested what would happen if one of the Primes shut down their libraries, but there still is a tangible sense of ownership with the Primes.
I actually have no problem with Microsoft dominating the Subscription market, but it should do it by creating the best platform, with the best features, not by gobbling up game makers, and then playing dirty with them. Which lets face it, is the Microsoft playbook...
(sorry for the long explanation)