I mean, people seem to have figured it out for Netflix, etc, and 1080/24 shouldn't use massively less data than 1080/60 or 1080/30. Even at a 1 TB limit and the extreme 20GB/hr rate, you could still get in 35 hours a month and have 300 GB left over, which is substantially more than most people use. Even though I have Youtube open in the background almost all the time, I only used 260 GB over the last 30 days, and something like half of that is game downloads.
I'm not saying this is going to be accessible to everyone, but they wouldn't be pitching 4K/60 and Doom if this was intended for the mass market. They're going after a specific audience that skews towards demographics with higher-than-average connectivity and income. And let's be realistic: Google knows more about how internet speeds, data usage, and interests overlap in the US than anyone else, and unless they're wildly incompetent, they checked that data to see if they had a market before they spent untold millions developing this.
They absolutely could be wrong about the interest level in streaming games, and I won't be particularly surprised if this doesn't become a particularly large player in the industry, but Google gets free speed tests of virtually every person in the country dozens of times a day. Their core business model is building as detailed a profile as possible on all of those people. If they're nonetheless wrong about something as relatively trivial as checking whether there are enough people interested in gaming with high enough speeds and data limits (which should be easy to estimate from your ISP, speed, and location), then a whole lot of analysts are about to get fired.