I think they're being sued over delisting someone for this last I checked, even if their public policy might not interpret their MFN that way

So does Apple. Despite this, they are both engaged in rent-seeking (https://en.wikipedia.org/wiki/Rent-seeking), which has a harmful effect on everyone but them.

Imagine if roads weren't public, but were built by a single private company. You have a business that moves goods by truck. You can use the private company's roads, but only if you pay 30% of the profit of your goods to the company that owns the roads. It only takes 2% of the profit to maintain the roads; the other 28% is profit (rent) for the road-owning company.

You could choose not to use the roads. But then the only way to deliver the goods is by parachute (which may be possible, but isn't practical). So you use the roads. But this means you have to jack up your prices to make any profit for yourself. Competing is much harder (tighter margins), and your customers are paying more than necessary. Everyone's life is harder, except for the road company.

Happier is a fine place to be. They are both still too high. Not everything has to be binary -- I can think Valve is offering some utility and also think that Valve is charging too much for that utility.

The fact that Gabe has a billion dollars worth of yachts probably suggests that maybe, just maaaaaybe, that 30% could be lower and Steam could still provide you the same level of marketing support and player base.