The insidious nature of Steam is the fact that they’re basically a PC gaming business monopoly,

Steam is a market leader, not a monopoly. Steam looks like a monopoly because it is the natural effect of being a market leader.

The fact that 80%+ of PC game sales are through Steam doesn’t seem to faze or disturb them, when it most definitely should

Reasonable disturbance but I personally will only categorize this as a concern.