The Ubisoft one is just particularly shitty. The only reason steam remains unthreatened is because the rest of them have mentally crippled shareholders at the helm. If steam ever went that way or undermined customer ownership they would surely be fucked and everyone with a functioning brain would go GOG or sail the seas. It's also not like they chose to or actively pursue being the only game store. The only limiting factor in using other game sources on the steam deck for example is the lack of interest from any other sale platform to support any degree of Linux. Open source devs have already replaced most if not all of their functionality with easily installable frontends. If one was really so deranged, even windows can be installed on the damn thing. I've at least never heard of any valve enforced steam exclusive titles, but vaguely recall some developer advertising something as only on steam.

The only way I feel one could justify calling steam a monopoly is to totally shit on the utter ineptitude of the competition so far as to dismiss their very existence, using it more as an insult to the competition than a descriptor for valve/steam, which is valid in a way but I don't think it makes the terminology usage objectively true.

Even ignoring all that or if steam actually was a monopoly by my or anyone's standards, I'm more concerned about the things that technically are not monopolies yet collude (even unintentionally which is unlikely) to fuck everyone over. Such as food industry globally, Canadian telecom, the current state of tv/movie streaming, etc.