The thing is that even with data caps being very realistic for people, games are just becoming too big to really have a sustainable model for those folks. Sure, you get to install the base game off of a disc, but that immediately comes with big day one patches at best, and if you buy it later, you could be in for files as big as the game itself. That is going to inevitably turn folks off with shitty internet, and the industry will probably just cut their losses. They'll base their model off of the majority.
I think Stadia is failing simply because it doesn't offer anything unique, not because of internet issues. It's essentially just another game service app you can get on any console out there. You'd have to be a real die hard Google fanboy to support it, over just getting Steam on PC, or a PS4/Xbone with the gaming services they offer. I have no idea what Google was thinking with that. They seem to be incredibly ignorant about the gaming industry, and will probably suffer some big losses because of it.