You don't need a subscription to play your games. You need a subscription to play them in 4k. 1080p is free. The main advantage of Stadia applies to people who don't own capable gaming hardware. I think Stadia will be successful. Google has too much advertising power for it not to be.
"At 10$+ a month, a 6year console cycle ends up at 6years x 12months x 10$ = 720$."
If you had a console you'd be paying for PS+/Gold which would mitigate some of this. Online multiplayer on Stadia is free, no subscription required, and with the subscription you will get some free games (Destiny 2 Complete at launch) though we don't know how many and how often yet.
As for the 1080p limit, well Stadia has 8TF of power if I remember correctly, probably around equivelent to a GTX 1080, if you're running games at their maximum settings you probably won't get above 1080p@60fps in most titles anyway, so your choice will be between Ultra 1080p or Medium 4k. So 1080p might actually be the better option anyway, especially as it won't require as much bandwidth.
Lots of people mainly play single player games. PS+ and Gold give you free games, so they pretty much pay for themselves. Anybody that feels they need Gold or PS+ year round should buy a year's subscription for $40 around black friday. If anything the $40 ($3.33 per month) for a year's subscription is almost too good of a deal to pass up, when you realize that you are getting $5 to $15 worth of free games every month.
The only reason I'm not subbed year round like this is that I already own almost everything that goes free on PS+ or Gold anyway. If the game has 79+ on Opencritic there's an 80% chance I own it.
But anyway, the old "Let's factor in the cost of XBL/PS+" argument is tired, and outdated. The days where XBL cost $10 a month just to play online, with zero other benefits thrown in, are long behind us.