"Google kills all their products" is a bit of an overused meme trotted out at any launch... But for Stadia it's a pretty legitimate concern, I'd say.

The model where you're still "buying" the games ties you to the ongoing operation of the service much more strongly than in many of their services where an eventual migration might be a little painful, but it's still possible.

The counterargument would be that they've deployed a ton of network capacity and GPU resources for cloud computing anyway, and so Stadia is just riding on those coattails... But it's certainly not a promising track record.