Current gamers already have gaming PCs, so there isn't much of a benefit.

Even for people that don't have gaming computers or consoles, you can put a decent one together that costs less than one of Google's phones

True, but the only PC you're going to build out that competes with the price of a Chromecast Ultra is a Raspberry Pi retro box.

Gaming PCs need to be upgraded on a 2-4 year basis if you want to keep near the top-of-the-line. One of Google's phones can cost $600-$1000, which is significantly more than a new console. So I'm not sure why you even bring that up, as if there were no economical difference between putting $500+ on your credit card, and trying out a $15-$30 month subscription.

Computer upgrades amortize pretty well. Over the last 8 years I've made 2 upgrades to my computer: A new SSD ($100 for 1TB) and a new GPU which is overkill for many purposes (~600 for a GTX1080). That setup can play many games at higher than 60fps, which beats the hypothetical max of Stadia (and I'll believe "4k at 60fps" when I see it— the Project Stream demo seemed to have a much lower framerate than that while still having noticeable artifacts on a 1440p monitor). Compared to that, $30 a month for streaming (if that's the price) seems as predatory as those companies that let you rent-to-own a TV. Although I'm not a fan of renting games vs owning.