Email has no default client.
TV has no default client.
The telephone network, after a lengthy battle involving AT&T, has no default client.
SMS has no default client.
Somehow, people find a way to use these things. It took many years from when the first web browser was written in 1990 to get to ubiquity. We got RSS in 1999 and it is in widespread use among web publishers; I can't think of a major news site that does not use it. Where we are lacking is in user uptake. Give it time. Open standards always take longer, but one will eventually get us there.