Google never had a chance to succeed because when Stadia was announced people were immediately saying "oh, I wonder how long until Google shutters this". Google's reputation for killing its services is coming back to bite it. It almost wouldn't have mattered how good the games they developed were, if people were scared to try out the platform they're on for fear of losing a lot of money and time investment.
Also they could have allowed any controller to work, that would have made lots of people try it, I know I would have done so, but if I need to buy a hardware that will be useless if the project fails I'm highly unlikely to try it.
If Google would have said upfront that all purchases would be refunded when Stadia shuts down, it likely would have increased confidence enough that people would actually use it.
Maybe, but money was probably not the only thing keeping people away, and a statement like that would have drastically decreased confidence that it would stick around, which may reduced take-up among people for whom other concerns—like the mental energy of spending time in a doomed platform may have taken—were more at the fore.