I suspect having a separate SSF ladder would actually increase the total playerbase in the game. There's a lot of folks who play early in the season, who can't be bothered to push leaderboards because they don't want to join groups to farm paragons, and with the massive advantage given by paragons, it just doesn't make it worthwhile to compete.
Not sure interest is high enough to warrant doing something like that at this point. Maybe a boost in player count and a severe push for SSF during Necromancer launch? Highly doubtful even then. That being said I'm 100% behind getting an SSF league like PoE. Either self regulated as you suggest or by design as it is in PoE. It would be an incredible addition to Diablo 3, and hopefully games of a similar ilk going forward. It's just unfortunate that I believe Blizzard's philosophy is still multiplayer first for 'X' reasons.
I think Blizzard's design philosophy of shoving everyone into the same narrow playing experience is one of the big reasons the PoE left Blizzard in the dust in terms of active playerbase. There are just many more ways to experience PoE than D3.