If Steam is the only place to buy your games from...
1. If Steam wants to screw over developers in any way they totally can, and devs can't do anything because there's no other stores.
2. If Steam wants to screw over
customers in any way they totally can, and you can't do anything because there's no other stores.
Just because they haven't done these things yet doesn't mean they won't, and when (not if) they do these things, would you prefer there be no competition or alternatives? If the tiny performance hit from having another storefront in the background is really your only reason for not liking the idea, just kill the program when you're done with it like any sane person does with any other program.
Besides, with Steam having no competition there's no reason for Steam to ever upgrade or get better or do anything besides exist and absorb the near-100% market share it's been enjoying for decades now.
My comp used to have just one storefront. It was called my bookshelf full of CDs.
And where did those CDs come from? Unless you pirated everything and then for some reason burned it, chances are you didn't buy them all from one brick-and-mortar store. If store A didn't have a game, I would to go store B, C, etc. until I found it. That's how a free market works, sans monopolies like Steam.
Furthermore that drop in cut, customers will never see that, only the developers.
And developers actually earning money = bad? Not every developer is EA or Blizzard or Epic, most indie studios live or die with every game they put out.