That still doesn't cover the cost of a high-end gaming machine. GPU alone can be north of $1k.
Imagine not having to re-buy all those components every 2-3 years.
Stadia isn't going to give you the visual quality of a high-end gaming machine so that's a bad comparison. 4k at 60 FPS doesn't mean anything if there is lag and compression artifacting.
Stadia is like a 960 GTX equivalent quality for the free tier and a 1080 GTX for the 4k tier. Let's not kid ourselves about the level of hardware power that one gets from this service.