The sandbox isn't the problem.

The missing authority the user has over it is.

E.g. you can have no wallet gardens but still sandbox every single application (which btw. I would prefer).

Which AAA games are web apps?

Kantai Collection ? ;-)

The ones being served by Stadia, xCloud, GeForce NOW and PS Now.

Plenty to choose from.

I don't know anybody nor recall seeing more than a handful of mentions online of anyone getting their playtime via those services.

The only demographic that matters in the long run is the youngest demographic, and that demographic is the most likely to own/use very underpowered hardware (Since they don't have their own money to spend on nicer hardware. Often they use hardware issued to them by their schools, or hand-me-down hardware from older siblings or their parents.) The specific implementations of Stadia/etc may not prove successful, but eventually I expect a service like this to overtake the alternatives for very young users, and subsequently, achieve dominance when those young users come to view those services as normal and familiar.

That's a pretty loose definition of a web app, but I agree that those services are pretty poor.

Well, one needs a browser to access them....

I guess I’m splitting hairs. At least these systems aren’t the only delivery method for games, but who knows what will happen in the future.