In what capacity has Valve squeezed their suppliers, though? They started out at the 30% cut from day 1, when they were one of only a tiny handful of players on the market, and it remains there to this day, only now if you sell above certain thresholds it's lowered to 25% (for big hits) and then 20% (for mega hits). They service games that are sold outside of their own storefront, both through retail channels and third party key resellers, sales on which they take no margin. They have increased the amount of services they offer to developers, not decreased it.
It's other storefronts that have come out offering lower cuts in an effort to entice developers to jump onboard, not Valve jacking up prices. Both from a consumer and developer perspective, Steam offers you much, much more in 2019 than it did in 2006, and the margin they charge developers has not increased. If you're a big player, it's decreased.
It really does not fit any model of how a monopolistic entity behaves. This isn't surprising though, because competition from PC Digital Distribution services is not their only worry. They're also in competition with consoles, smartphones, and browser games, so even if they had 100% control over all PC digital distribution, it's not clear that it would be necessary for governments to take antitrust action against them, because they would not control all of the commodity or supply since "PC games that you buy and download" isn't a class of commodity in the same sense that controlling all the milk or oil in the world is. That's also why regulators don't break up Sony just because they "have a monopoly of games sold on playstation". It doesn't matter, because Playstation itself doesn't represent all videogames.