But unfortunately, this isn't a problem that Steam can address and it's fully under the responsibility of the game.
If Steam banned external launchers, a lot of games would need to retroactively fix itself. And I can also see future lawsuits as making it appear as non-competitive.
Agree that they can never fully address it, but it would be nice if they made it easier to block publishers and developers (who do not have a publisher or dev page set up, like Sega) and filter on things like "Requires 3rd-party DRM" that appear in the gold boxes in the Steam UI. Currently, I follow multiple curators who flag games for things like Denuvo. But, it would be nice to have that built into the filters and store preferences, when the info is available. If users could easily filter out bad actors, then it might discourage the bad behavior. Valve might not do any of that because it would probably strain their business relationships. So, I don't know.