You say that you don't think the unusually high number of living presidents will last long, but I think it will last for a while yet. Since you wrote this, G. H. W. Bush has died, and Carter is now 95. However, the next four are all 73 or younger. If they all live to, say 95, then after Carter we won't have another presidential death for 20 years, in which case we will almost certainly see 7 living presidents for the first time, and possibly more.