Git or github isn't the answer to this. Why was SMF successful? Not because "everyone" could contribute to it - but because a small team was allowed to work their dream out. If everyone could participate code-wise you get splintered code, if everyone could have a say in what to add you get endlesss discussions, if everyone should decide how it should look you get..well, not sure what you get then lol.
Its always been about this: that a small team follow their dream and perhaps fight among themselves in a healthy way, NEVER to be decided by the supporting team. If [Unknown], Grudge, CompuArt, Jeff hadn't been able to work limitless in the beginning of SMF - would SMF have been this popular? I doubt it. The support becasme important later on, it should and could not be a key factor DURING the development.
I believe thats a crucial thing to remember. Then you of course can argue WHO should take on something like SMF, since all the first-starters are long gone. Which is why people like Thantos, JayB etc. got to shine too, in getting SMF 2 going. Again, small team. I was lucky too, to work on the graphics and I think SMF was the better for it.
So Github for SMF is not a solution at all IMHO. Unless its not important at all, where SMF is headed. Is it?