I wouldnt count on the winter weather breaking up the large groups (NYC, etc..).. there will be some of them that attrit.. but I seriously doubt it will be the majority of them.. its going to take something other than some snow and some 15 degree days to run them off..
they are being well supplied by different funding sources.. they can afford propane heaters, insulated tents, insulated sleeping bags, cots to get them off the ground, winter coats, etc..
I agree with you.. wet sucks much more than cold.. when you and everything you own is soaked to the core.. and there is no way to get it all dry again.. thats when you'll start seeing people "break down"..
All that really means is they will go back home for a week (do this in shifts), dry out, eat well, etc.. then theyll head back out to the camp site..
Let a couple of those propane heaters set a tent city or two on fire however.. and the game gets changed..
it all really depends on how well (or poorly) they manage themselves.. as to whether or not they can hold out..