Assuming they give the controller kit for free, Stadia costs $10/month, so in 4 years you'd have paid $480, almost the same as a new console. plus, the price of the console likely isn't an issue if you can afford buying a couple of games that cost $60 each.
then you have the latency problem in Stadia and the fact that you can't really play it from everywhere (even though it's in the cloud); you still need the chromecast, right? they might as well beef up the chromecast and make it a regular console. the newest chromecast already can play android games I think.