Is it though? A new game console usually costs around $300-400. And that will likely last 5-7 years. Stadia Pro is $10 a month (right now). So after about 3 years you've paid for that console. Even if you factor in the coat of the PS+/Xbox Live Gold ($60 a year) over the life of the console, you're still spending only breaking even at best (and those services also come with free games every month as well). If you buy the Chromecast Ultra and the Stadia controller, any potential cost savings becomes even less.
Now, you don't HAVE to get the Stadia Pro subscription. But without that you're limited to lesser performance in the games. I suppose if you don't care about reduced graphics or frame rate at all, then maybe this would appeal to you.