>browsers don't really have a good incentive

Why wouldn't money be an incentive. If businesses are willing to pay to have locked down browser access their cloud files, and the cloud file website wants to make money by charging businesses for this feature it makes sense that they may pay a browser to develop such a feature to use with their website.

I dunno, it just seems like the set of circumstances that would be needed to overcome the inherent friction in a "healthy" ecosystem is a lot more gymnastics than the current situation where the browser company with the vast majority of marketshare is the company that has conflicts of interest to fuck with the browser.