RE: UIG/PYP, they were closed along with everything else because we wanted to really be able to empirically say whether a giant thread is actually worthy of being around or not. Just closing a few and not other would've led to cries of "aw you closed this thread I love! But you left thatone open. NO FAIR!" which wasn't the point.
I don't really know what you mean by 'more lenient' because currently we're the most relaxed we've been in ages concerning new threads. So long as they're not directly replicating mass threads then we're letting it slide, pretty much regardless of content/potential/quality.
Paj; a massive music thread would go against exactly what you're saying, and a sub-board would not alter it at all. As it currently stands, GCG would simply be less cluttered (a point you, amongst others, have made, I think?) if such threads were 'subbed'.
I also think not having the 'numbers' is fairly irrelevant to the argument, because wherever any of these topics live, the viewership will not increase or decrease. A sub-board could essentially reduce clutter and provide a focus.
I don't really understand how such a shift would create additional barriers, besides the meta-coop the threads would be contained in...