To argue that a separation of the two forums (old and new) would cause a lowered development focus for the legacy OS would, I think, make most 770, N800 and N810 users smirk a bit these days as we are already the neglected ones. This way at least we might have a forum that is readable
But since it has proven impossible to get such a structure in place here at tmo I think I am naturally cautious when thinking it would work 'over there'.
Also, remember that with MeeGo it is no longer an OS dedicated to Nokia devices. There may well be 10's (more?) of devices running MeeGo in a couple of years. I wouldn't want to rely on 'tags' to search such a forum for, say, "N810 applications" and I don't think that it would make sense to have every device as a sub-forum under each category (app, etc).