I've been using GeForce Now with a Nvidia Shield for over a year now and it's been a great experience. The hardware costs are the same as the Stadia ($130), it's free to use while the beta period lasts, and you can play all the games you've already purchased on Steam, BattleNet, Epic etc. in 4k.
Why would I buy all the games AGAIN on Stadia when I can access them on GeForce now? The social/streaming aspect of Stadia is not a big draw for me either, so I really don't see the advantage from my perspective.