First, you have people who are "real" gamers. Whatever that means, but we'll define it as owning some piece of hardware that was bought for the purpose of gaming (PS4, Xbox, Switch, PC). This number is between 250-500 million people.
Yes. Its that high. PS4 sold ~115M units. XB1 ~50M. Switch ~70M. Steam has ~100M MAUs. There's overlap in those, and those numbers also really under-represent China and the huge PC gaming scene there, but I think we're in the right magnitude.
By and large, this group's interest in cloud gaming would only significantly extend to being able to access the games they already play on the hardware they have, when out and about. Xbox, Nvidia, PlayStation, and whatever Valve releases will own this group. They're not re-buying games on Stadia or Luna and locking themselves into that platform, nor dealing with locked cross-saves, cross-achievements, cross-social, its just not happening.
Second group: Lets say iOS users. Representing 1.5 billion people worldwide, significant overlap with the previous group. These users probably wont get too heavily invested into any cloud streaming service, because they are all inaccessible on their mobile device. Yeah, there's a Luna web app: I'm sure it'll be horrible.
Third group: People with bad or no internet. 160 million Americans lack broadband. Some of them still buy game consoles and suffer through weeks of downloading updates. Most don't. Online play sucks, but 150 ping is manageable. You learn to play with the ping. These people will never use any cloud streaming service.
We could go on, but the point I'm reaching is: I, too, have no clue who will be signing up for these services. It really feels like these cloud companies just have graphics cards laying around and are supremely uncreative with how to get value from them. Or, they want to own the world and its taken them thirty fucking years to recognize that, hey, gaming is a thing and boy are they late to the party.
Gaming hardware is not expensive. A Switch costs $200; a brand spanking new Xbox Series S, $300. You don't need a two thousand dollar PC to play all the great games out there. Stadia and Luna want you to buy their controllers and their dongles, the upfront cost is not zero, and consumers are not blind to the ongoing cost and expensive games (Game Pass anyone?! Its a fucking steal! And it practically comes with xCloud! Why do people still think Amazon and Google can compete with this?)
If there was some investment vehicle which allowed me to specifically short Stadia and Luna, I would put every cent in my savings and retirement into it, down to the last penny. Cloud streaming has a future, but it won't be via these two products.