GTF, true that the market will reach equilibrium. The main issue is that, including Rails to Trails which held its inaugural running a couple of weeks ago, there are 5 new marathons in just over a 6 month timespan. This growth rate, with an already full schedule, is one that just doesn't seem to be supportable. I fully expect at least a couple of these to not be around in 2010 and I'm specifically looking at the May schedule.
Rita, Madison actually has had some issues in recent years and I've heard from some that Fox Cities is less than it used to be. I'd be highly surprised to see any of the three you listed go anywhere but, if I were to pick a dark horse, I'd say Madison isn't as safe as some on the schedule. Partly because it's part of the May gauntlet and partly because of some issues it has recently struggled through (some of the organizer's own making, some just bad luck).
Actually, the nice thing about the Wisconsin marathons, in my opinion, is that none of them are megathons. The largest (Lakefront) has a limit of 2500 full marathoners plus a relatively small number of relay teams. Maybe that would help support the number of options. The size of the events means that people can spread out and try different ones. Even if we figure that the average event has 1000 runners, that's still fewer total runners than Minnesota's two largest marathons alone draw.