"We stream drawing primitives instead of heavy pixels or complex state objects." This is cool ... but I suspect just pushing video frames like Stadia etc did is just as efficient these days and a lot less complicated to implement and no special client really needed. Decent compression, and hardware decode on almost every machine, hardware encode possible on the server side, and excellent browser support.
On the other hand, you could take a list of primitives from, say, the JS Canvas API, come up with a format that can encode all of them and use that as your protocol. Bam, with that you get one client for any game that uses JS Canvas.
That said.. I don't think stadia could do that since it's not opinionated about the game engine. Unless they go really deep on the graphics card instructions instead, but then it becomes comparable to pixel rendering I guess