Well, it doesn’t really do that. He doesn’t really have a particularly impressive performance compared to the other cards in decks like ramp druid, compared to say, control warrior, where he is literally one of the best things you can do.
The win conditions in the current good decks that are going late enough to run Theotar are much, much stronger than Theotar is himself, so for the decks that have those win conditions, he’s basically just a backup card to recover from something being stolen, or to prevent the opponent inevitability from happening first.
For decks that don’t have as decisive of a plan, Theotar helps to make sure you don’t have a 90% loss rate to ramp druid.
None of the control decks have much of an issue with having a sizeable hand. The rest of the field generally should be playing aggressively enough so that Theotar can’t safely be played just in case you have a relevant card to steal at that time, so the size of your hand matters far less.
In most decks using Theotar they also have to be careful when they use him so that they don’t have to give you anything you can use well. I’ve had a great number of opponent Theotar swaps that ended up in my favor.
He does punish more passive decks more harshly that rely on cards that you have to hold and also struggle to keep the hand size large, but that’s not exactly a huge number of decks, and those tend to have pretty bad consistency issues in the first place that keep them from being particularly playable.
So, extra harming bad decks isn’t really a flaw of Theotar. He’s not the reason the deck is bad. Decks like those tend to be hyper weak to other things too, like silences, board clears, things that go tall/wide, etc., Simply because they struggle to keep their hand full of options while relying on a swing card to carry the game for them.
