I'm not the only one who thinks that Worldcon committees have got to figure out some way to do joint marketing even if the marketing doesn't seem to have any direct effect upon their own Worldcon. That's pretty difficult, but we did manage to find a way for Worldcons to do revenue-sharing with each other. Any serious joint-marketing effort will probably need the equivalent of Noreascon Three to get started. (N3 jump-started Pass Along Funds back in 1990 by donating money to its successors even though it received no money from its predecessors, in an act of generosity and an interest in Worldcons as an whole even though it was against MCFI's narrow self-interest.)
Incidentally, if Worldcon attendance declines much further, the convention will get cheaper because we'll have shrunk back into hotel space and won't need to rent convention centers anymore. OTOH, if the price went down that dramatically (and it would), the demand might cause it to thoroughly overload that hotel-based facility.