There's an easy product fix for this. Offer every game purchase with the option for a one-time key conversion, ideally managed through the game publisher. In the event of a Stadia shutdown you could easily port your purchase over to another single platform of your choice. This stragegy de-risks the platform and would go a long way toward alleviating consumer fears. We saw similar strategies around the launch of Steam…
The problem is that, to move them to a new platform, for example Steam, they need to pay the platform's cut of the sale price. To give users the option to do this in future, they would need to keep all of the revenue they get from selling games in reserve to possibly do this in future. They would have to do this by charging users what it cost. I think it'd still be attractive though.