The last Worldcon in "the Midwest zone" was in Toronto in 2003. And, to be honest, Toronto is as far east as Pittsburgh and almost as far east as Buffalo, both of which are clearly "Eastern zone". The only reason Ontario's in the old "Midwest zone" is because of the large, much more sparsely populated section of the province that overhangs the Midwest. But I wouldn't bet on a Thunder Bay Worldcon ever happening. So by geographic measures, the last Midwest Worldcon was probably Chicon 2000. Wow!
It's definitely been the "screw the dealers" decade for Worldcon. Since ConJose in 2002, the Worldcon has gone to Toronto, Boston, Glasgow, Los Angeles, Japan, Denver, and now Montreal, with a dead certainty that the next Worldcon will be in Australia, because the SMOFfish contingent would be sure to punish anyone who dared bid against it. Three Worldcons out of eight on the right side of the customs barrier.
If you're making your living from your fannish business, you might as well just start planning on going to Dragoncon. (We don't and I don't.) Who knows? Maybe that was part of the SMOFfish objective too. I've heard this constant background rumble that Worldcons had gotten to be too large.
I guess that's been fixed. Heck, even the most recent LACon was much smaller than we might have expected a few years ago.