Probably the largest and most influential group of liberal Catholic believers is the Voice of the Faithful. They seek to reform not just the church's position on specific policies like abortion and gay marriage, but also more fundamentally in matters of organization and control. The problem is that Voice of the Faithful is primarily older and white, which is not the demographics of the church's future -- neither in America nor globally. They won't get anywhere if they cannot attract a membership which will endure.
What's more, they also won't get anywhere without a membership that is passionate. I don't mean to suggest that the current membership of Voice of the Faithful and other liberal Catholic organizations don't really care, but are they as committed and as passionate as members of traditionalist organizations? Are they as willing to fight and sacrifice as traditionalists? I don't think it's coincidence that more conservative and traditionalist Christian groups across many denominations have generally done better over the past decades than more liberal and moderate groups.
Liberal Catholic groups are fighting an uphill battle in more ways than one: they are fighting centuries of entrenched conservatism, they are fighting forces based primarily in a distant land, they are fighting social and religious forces spread across the entire planet, and they are fighting a legitimate orthodoxy. Making the fight harder by having a narrow, limited, and even dwindling base of support isn't a good idea -- but how easy would it be for them to broaden their appeal?
As a former Liberal Catholic who had hoped to change the church just a little from the inside out. I learned quickly that most Catholic I knew & met were fighting hard to go right & as far from compassionate left or even middle as possible. They wanted nothing to do with caring for people only the steel boot rules of pre Vatican 2. These Catholic rejoiced in the Bush Era war/fear/intolerance. It was suffocating. The very few other liberal Catholics I have met have left the church or are still beating their heads against the church doors.
I don’t think you should discount the fact they could end up being silenced, as well. Excommunication (or the threat of it) has been used as a weapon to silence dissenters for nearly the entire history of the Catholic church. If they ever gain any traction, expect this to happen.
There will be no moderate or progressive reform from within the Roman church. This reform MUST take place outisde the sphere of influence of the bishops and the Vatican.
That is why I support the formation of an American Catholic Church governed by independent Catholic laity and clergy.