Lag really depends on network connectivity and how close data center is. I get very low lag for geforce now. It's so good that lag isn't issue for me when comparing something like cp2077 local vs. cloud. What kills it for me though is bandwidth needed and data caps. I only get super blurry 720p. I don't see any reason why stadia couldn't be low enough lag.
What kills stadia for me is google's history of killing services. If google kills stadia one would lose all the content. I rather bank on something I can keep using on desktop pc if service is killed.
Stadia is not a competitor for the PC, all of it screams"CONSOLE".
1. Lag
When you run you game on a PC you avoid the controller lag on consoles (on top of the network lag)
So now you combine controller lag with network lag, time from click to frame on consoles (the "next-gen" ones) are ~75-130 ms:
https://displaylag.com/console-latency-exploring-video-game-input-lag/ )
(data does not travel at the speed of light in a cable, it is about 2/3), meaning you will have to dot the world with datacenters to avoid high lag.
And tests do show this:
https://www.pcgamer.com/heres-how-stadias-input-lag-compares-to-native-pc-gaming/
The lag is horribad!
2. Settings/modding
You are locked. Again this screams "CONSOLE"
Both visually, but also audio (I game on 7.1 speaker setup, 5.1 has poor 3D spatial mapping compared to 7.1 IMHO...those side speakers ^^)
And if you want to enchance your gameplay...forget about it...no modding.
Mods are one of the things making PC gaming superb in my book.
3. Adding more failure points
Now your ISP and the streaming servers are added failure points, meaning a network failure will spoil your singeplayer game.
5. Game ownership/locked to the provider
I have a large games collcetion.
As far as I know, you cannot transfer your excisting games to any streaming service. Meaning I will have to buy games again.
And I will have hope I pick the right provider as games do not transfer.
The games I own today...I can keep playing for as long as i want to.
And the service is walled garden (one of the reasons I despise Apple)...I am at the mercy of the service.
5.
Old games.
How long will a game be available?
I doubt any services will keep supporting games for as long as I can play them on my local PC.
Again it is like the consoles, where back-wards compability is a joke compared to on the PC.
I'd had better get some work done...but I do not see streaming gaming as a threat to the PC...it is a downgrade, the same way console gaming is compared to PC gaming.