I fear having to keep a separate launcher for every major publisher in the future.
I think having something like Steam is great primarily for discovery and ratings.
What would make me be a fan of being agnostic would take something similar to Movies Anywhere, where if I buy a movie on Vudu and it automatically shows up in my Movies Anywhere "library" or more tangibly, in my iTunes, Microsoft Movies & TV, and Play Movies libraries.
Prior to Disney Movies Anywhere, I would forego the deals on Vudu due to being yet another library to manage. After DMA, I started to buy Disney/Marvel/Pixar movies where the best deal was at. After Movies Anywhere, that expanded to titles that were part of Movies Anywhere.
Imagine having to manage all your phone's apps through different stores, launchers, and website downloads. Sure, you can. But it's not as seamless as having all your apps in one place.
Maybe I'll see Half Life 3 in my lifetime after all.