To maximize sales, you do need to put your product on Steam. That doesn't mean it is a monopoly.
Steam likely could leverage their market position to become a monopoly, but they aren't one now. Just because they are a vital part to any large scale game launch doesn't make them a monopoly, quasi or otherwise.
I don't think it is a particularly healthy market, and I wish there were even more options. That doesn't change the fact all the monopoly talk has been driven by bad faith actors trying to attack steam without any real cause.