At least for myself, if they did something like this I would cancel my Stadia preorder immediately. I'm less interested in the actual game streaming itself and more interested in the applications of that when it comes to exclusive games, scaling using the data center, giant mmos with no lag, etc. The potential is incredible and if Google allowed any kind of downloading it would show to me they aren't serious about creating games like that and I'd be out.
So you pre-ordered the service based on some idea of future games they haven't even announced that will allegedly be so advanced they require Stadia's data servers? That's pretty odd... they literally haven't announced a single title like that.
But again.... my idea doesn't stop Stadia Exclusive games from happening.. just the other 99.999% of the games they offer would have a PC copy.