I don’t think cloud gaming will replace anything for a while, right now it can work as a supplementary option, which is why I think it’s going to be better for Xbox than it will for Stadia–With Xbox you can still download and play any of those games, so you aren’t limited just to playing a game over streaming. Besides that, they have a full system of integration, so you can play your game installed on the console or PC and then if you’re traveling you can pick up from where you were playing in the game using the cloud service.
For now, it’s a really nice extra feature that allows you to have more flexibility to be able to play your games, but with Stadia that’s all they offer and I don’t think many people want to buy games that way.