It's still $10-20 forever, monthly(assuming they never increase prices). Versus a few larger sums every few years. If you don't buy the latest and greatest, a $200 yearly budget can definitely keep your hardware up to date. If it's a PC, you can even use it for other purposes.
Let's do a little math. Let's say you want to buy a gaming PC and have a budget of $1000. Let's also assume you play 1 hour of video games a day, 5 days a week. That is 20 hours a month.
At 20 hours a month you would have to play for 50 months (4 years) on your machine before you pay off the machine. Alternatively, you could spend that same money on the streaming service. I think the math doesn't hold up if you are a casual gamer. Building a PC and maintenance just isn't worth it.
That being said, if you play a lot more, maybe even 15-20 hours a week, I think it makes a lot of sense to build your own machine (I have one and it was very worth it when I was a serious LoL player).
Within the context of Stadia here, you could purchase a game at cost and play it forever (or, until the service shut down, heh) without paying any monthly subscription fee, which does change the math somewhat.
