It mainly depends on two things:
1. How serious is Google about Stadia? If they approach the market like both Sony and Microsoft did when they entered the console market, they will buy their way into it. Google can pay for deals to get either exclusive games or have big multiplatform titles playable on their service a couple of weeks before they release on PS and Xbox. Additionally, they can buy the exclusive marketing rights which has been common practice for over a decade now. Google certainly has the money and infrastructure to be a competitive force.
2. How fast will the market embrace streaming, if at all? That's a big question and it's likely to stifle the growth of the streaming services of Google, Sony and Microsoft, so how aggressive Google will run Stadia could ultimately be moot.
As for costs, $720 for six years comes in considerably lower than what Sony and Microsoft consoles do. Being generous and assuming $400 price tags...
Console - $400
Mid-gen upgrade - $400
Online subscription - $360
Total - $1,160 or $800 if someone foregoes to play games online.
Google's version of the mid-gen upgrade is that they update their server blades and that will come at no cost to the customer.