Mandatory preface to prevent angry fanboys stinking up the replies: I like Steam. I use Steam. And just to be sure, democrats and republicans are not the same.
Some folks in this thread are using American case law to argue that Steam is not a monopoly, or that Steam is a good (??@#!?!?) monopoly. They look at other cases, like Microsoft, and point out how far Microsoft had to go before it was considered a monopoly by American judges, and then point out that Steam is not as bad. There are two problems with that line of reasoning.
The first is that monopoly law has been absolutely gutted by Reagan, and worsened by every administration (dem and rep alike) up until Biden. In the Biden admin, Lina Khan has made some very small steps to tighten up monopoly laws a bit, but obviously Trump happened (although Harris was pretty much the same as the dems before Biden, so not much hope there either). The bar for being a monopoly is unreasonably high, and American monopoly law is an absolute joke.
Secondly, this line of thinking conflates legality with morality, or being good (enough) for society. I hope I don't need to convince you that this idea is false. Slavery was legal.
The argument here is not that Steam is, in the current flawed legal American sense, a monopoly, but that it is a monopoly in the sense that it has cornered enough of the gaming market that it could do very serious harm.
Note that "they're not currently doing harm" is not a great counterargument here. When my neighbor buys a bazooka, I won't be satisfied by "don't worry I'm not currently using it".