To add to what you have said:

Valve is an effective monopoly.

A lot of people seem to think 'monopoly' means 'literally 0 alternatives for the consumer', but this is not the case in either actual economic jargon/theory nor in basically any legal definition of it I am aware of.

To be a monopoly you basically just need to be the clear dominant actor in some market. Not the only one, just the main one, such that you can make pricing decisions in a way that other actors in the same market can't, basically.

Its... very rare for a 'true' or 'perfect' monopoly to ever exist for basically anything other than a public utility/service. It almost never happens.

This is the kind of pedantry that is annoying but unfortunately important, similar to how 'Impeachment' by the House on its own is actually pointless beyond a mark of shame unless it is also followed by a 'conviction' by the Senate.

You are correct that in US law, a major factor that is considered is whether or not the company did abusive, deceptive, underhanded stuff to achieve its monopopy status.

But UK law appears to be different:

https://www.lexology.com/library/detail.aspx?g=c5b1e681-5fb5-4161-bebf-823034fab751

You could be doing 'abuse of dominance' whether or not you achieved that dominance by underhanded means.

So... while I am not a lawyer, I would be genuinely surprised if Valve was found in serious violation of existing US monopoly laws, but I would be less surprised if they were found to be in violation of existing UK monopoly laws.