Of course, if we upgrade to king size, then we'd need to move other stuff around in the room too. But the main reason for not going king yet is the cost of admission for something of good quality.
Even a new queen would probably be better than the current one. The ex-wife broke the box spring by jumping on the bed and I've had a sheet of plywood supporting that side of the mattress for nearly four years. Sheet of plywood: $25 (and I got my parents to give it to me for free for my birthday). New box spring: $120.