Yes, Steam is probably a monopoly. No, I don't really care.
I've been a gamer for over 30 years.
My first game was king's quest on a 386 PC in the early 90s. I remember the rise of consoles as a staple of the American home, and the dark days of PC gaming from the early 2000's.
Any gaming magazine you bought or website would visit would claim the same thing - that PC gaming was dead, and consoles were the future. This went on for more than 20 years (PC Gamer has a solid article with some examples of how many times PC Gaming 'died' in the 2010's alone https://www.pcgamer.com/the-13-times-pc-gaming-died-this-decade/ )
Valve was one of the few companies willing to keep the boat afloat during this dark period, providing the medium with the similar benefits that Apple provided to the music industry with iTunes, namely convenience, centralization and standardization.
While companies like EA were putting out atrocious console ports on the PC, Valve was delivering classic PC-first titles. They could have easily shut down the PC business and joined the rest of the companies as a publisher on the console side, releasing Portal 2 as an xbox One exclusive or some other sacrilegious pact, but they didn't. Because they had a vision, believed in the platform, in their product, executed, and it worked out for them.
We now see companies like Capcom selling more copies of Resident Evil 9 on PC than PS5 and Xbox combined. And yet, the companies that are now complaining about Steam being a monopoly are some of the same companies that were more than happy to abandon the platform altogether when it suited them, doing the bare minimum on PC, releasing terrible unoptimized console ports that barely ran. The same companies that were unwilling to put in the work when it mattered.
So yes, Steam is probably a monopoly. And no, I don't really care.