If you PC game and want your library on the go (on an Android phone - no iOS love as per usual), then I can see why GFN would appeal.
But if you have no Steam library and don't game on your PC/Mac then there is literally no incentive to use GFN.
Stadia will be fine.
No incentive?
The free tier is out already.
The subscription is cheaper.
It doesn't require the purchase of proprietary hardware.
The game library is far larger.
Games can be purchased from a number of different store fronts, leading to price competition.
Game prices are (generally) lower.
Games are effectively "cross platform"; buy the game once and you can play it on PC, Mac (game dependant), Geforce Now & other PC-based streaming services. This also means your library isn't rendered redundant should Geforce Now get shut down in the future.
Not to say that Geforce Now is better all-round. With Stadia you get:
4K resolution.
Potentially lower input latency with the Wi-Fi controller.
A more integrated, user-friendly, experience. Geforce Now is a bit of a hack. You have to manually search for games you own. After selecting a game, you basically connect to a virtual desktop, where you log in, "download", and then launch the chosen game.
I'd say there's more incentive to use Geforce Now if you have a PC game library, rather than less incentive if you don't. The incentives look pretty good even without a catalogue of games to fall back on.
My main disappointment with the final release of Geforce Now is that the UI hasn't become like the GOG Galaxy one. I want to be able to log in to all compatible PC platforms and see all of my (compatible) games. Having to search and manually add games to the Geforce Now library is laborious.