If I as a game dev have to list my games on steam due to their overwhelming market dominance, they have a monopoly. Whether people like to use a platform or not is not even relevant for the question of monopolies.

I use the term monopolistic to describe companies that have such a overwhelming market dominance that free market effects barely apply anymore. That is not the legal definition but a more useful definition that is popular in more progressive circles as it can be used to actually describe actual real world companies in the 21st century.