So "too big to fail" or something?

I don't know if you lived through the Internet Explorer era, but that was exactly the same situation in browsers back in the day. Internet Explorer was preinstalled in every Windows computer, so in pretty much every computer, and it was deemed as "unbeatable" because people were too lazy to install anything else. In retrospective, it didn't take too long for Google Chrome to beat IE market share and nowadays pretty much the whole world uses Chrome and nothing else. Now, with IE, EU had to step in and force Microsoft to present their users a dialog to choose their browser in a fresh Windows installation which did have a role in that market share change. With Steam there isn't a need for that, because every user has to go and explicitly install Steam client to their computer before using it. Same goes with Chrome.

Although, vendor lock really is a real issue, and I do agree with you that if one has thousands of euros/dollars worth of games in their Steam account, it's purely convenient to keep on buying their next games on Steam as well. What I don't agree with is, that if there was a new competitor that was better in every way imaginable and they were able to sell the games on their platform for, let's say, -5% constantly, people wouldn't start using their service. You have to remember, that there is also a constant stream of new gamers (young people) that haven't even created a Steam account, and nothing is preventing them from choosing another service for their first game purchase. It's just that there isn't a real alternative to Steam currently.