This sounds terrible, but it seems to me that nobody will let it get that bad. It's infeasible to support 70+ browsers simultaneously; no dev shop can afford that. So they would either pick versions or choose some common set of features to support. If this meant that IE users got crappy experiences, despite developers' best efforts, people might finally start ditching IE. (A man can dream, can't he?)

But I think the IE team will find a way to prevent this.