Basically people are upset because Steam has been good to them. They introduced the 80% discount after all. They opened up classic gaming and got devs to update a lot of these broken games for new PCs again. They opened up the indie market. People love steam and Valve. So they are kinda fine with letting them have a monopoly.
However, I think they are largely overlooking the fact that Valve has gotten lazy. They stopped releasing games from their main franchises. Their biggest accomplishment in the last couple years is updating their chat client to STILL NOT COMPETE WITH DISCORD... The few games they have released are a step above flash and Wii party games.
In short, their monopoly is unhealthy for steam. They've gotten lazy.