It seems to me that Coinbase alone could decide to make a URSF defacto "popular" whenever they wanted to - and the majoriry of miners would almost be certain to follow. It would be at that point tha...
PS. Short of overwheming consensus support from the community as a whole, I don't see Coinbase (or any other regulated entities) being particularly keen on tolerating ANY potential for a soft fork to cause signifact disruption.
