I'm not sure if I fully understand your point. In this solution browser is not expected to vault card. It should just be intermediary between payment methods and payment gateways (of course saving card inside browser can be one of the payment methods).

Also depends what do you mean by more mainstream. Google Pay and Apple Pay both support tokenization. So do many other payment methods - I would say that it's mainstream. Next step would be refusing plain cards at all.

Google Pay is everything that Google needs for Chrome to have full solution.