How would you even "run the numbers"? How about try the idea and see if it works? All it would take is one hit game for devs to see the potential of web gaming (and they already see it. Why do you think flash games were so popular?) If Fortnite could run in the browser it probably would have instantly become the biggest game in the world.

Fortnite will absolutely be able to run in the browser, once WebGPU ships.

We kinda had that before with Flash I guess, although the tooling was a huge on-ramp for that ecosystem.

I don't see any modern tools that are nearly as accessible.