

Valve does nothing to suppress competition (it can’t really either)
They at least used to have a rule that publishers can’t sell cheaper on other platforms (outside of timed sales that is), meaning that consumers can’t get a better price on other storefronts even when those platforms would take a smaller cut. That was very much suppressing the competition as them taking smaller cuts can’t transfer into cheaper prices if the publisher also wanted to sell on Steam.
That’s not enough, still. It should directly affect the rank too. Get low on behavior score, not only everything they already do, you will also start going down. Because rank is everything to those toxic people, it is the only thing that drives their toxicity.
The problem with mobas, and most modern multiplayer games, is the visible rank system. Back in the day, with only community hosted servers, people were playing for fun. Then multiplayer shifted to publisher hosted servers where you started playing with one click on the menu. To keep the matches interesting, you needed a system to matchmake fair lobbies, so here comes the ranking. But they made a mistake by making the rank public, because now the players are no longer playing for fun, they are playing to raise their rank - and losing even lowers their rank.