There are not many objectively provable monopolies and i doubt that English law would support that claim without extremely strong evidence, generally utilities are the only ones that'd get close. A necessity with high fixed costs and infrastructure lock-in.

Steam has high market share in a segment, but not necessarily a distinct segment, I'm sure steam would argue that there are enough consumers who can and do substitute between pc and console and mobile, as well as other vendors so that their market power is mitigated by a fair amount of consumer mobility.

So what you're looking to prove is unlikely to be a pure "monopoly" but 'excess market power', and 'abuse of market power'. That is a complex legal art that the competition regulator is usually not that successful at proving, at least in English law.

Abuse of market power has to impact consumers not producers. There are always marginal producers struggling to make a profit - that happens in competitive markets, producers bidding prices down, some going out of business. I'm not saying I agree, but that's more or less how the law sees it, lookup what they let supermarkets get away with in contracts with farmers.

To show consumer harm from upstream market manipulation you'd probably have to show a material dearth of choice being created by steam policies in order to jack up prices. Maybe that can be demonstrated, but it's not simple and more likely to come down to subjective interpretation of the arguments and evidence from both sides rather than any unarguable objective truth.

If it were unarguable or objectively true then the CMA might lead the investigation itself instead of this being a private action. Though maybe this is too small a market for them to worry about.