This. So much this. Monopolies are often evidence of an unhealthy/stagnating market, but they're more symptom than cause. Trusts/cartels, price-fixing, and anticompetitive behavior are the actual abuses of market power, and are much more problematic.

I'm not going to claim that Valve is perfect (they're not, e.g. see issues regarding DRM) and I'd love to have more choices about where I buy my games, but I can't think of any instance of them abusing their position in the market to prevent new entrants or claim an unfair advantage. From what I've seen, they appear to be a very fair and honorable competitor in the space. However, if anyone is aware any examples to the contrary, I'd love to hear about it and update my opinions.

Yes, but by pushing back on monopolies in the first place, you ensure that there are other options if one turns sour. Steam is great for now in a lot of ways. That can change - and if it does, we only really have GOG to fall back on, and their platform isn't nearly as mature as steam.