What's wrong with just downloading a patch and pointing it to the directory the game was installed in?

Nothing, but you can be sure that's not how it's going to work. A simple example with Steam and certain games: you have one launcher with Steam, but you have another launcher between the game and Steam, and I don't see why that's necessary.

But the way you want to do it requires a bit more work, but nothing too complicated, and more and more people want something simple and easy to use, which is not the case with your proposal because they have to do more steps to be able to play.