The mammoth and the earth elemental both have 126 HP, the air elemental (the druid's best flying form) has 90 HP. So the druid has a 130 HP/round buffer on the ground and 90 in the air.
The ancient red dragon (CR 24) can do about 90 points of damage per round, maybe 100 in its lair. That won't make for a very dramatic fight. I don't necessarily have a problem with a 20th level character single-handedly killing a monster of that caliber, but it should at least pose some level of threat, right?
If my campaign ever got to that point I would probably rule that the druid couldn't return to any form more than once per long rest. (but once per rest let the druid return to only one form a second time, so that the archdruid ability is never worse than the basic wildshape)
For example the druid could go mammoth, mammoth again, earth elemental, air elemental, water elemental, fire elemental, and then on to lower HP forms. (because he already used mammoth twice, he can only use forms once from then on). Naturally I would assume that there would be an infinite number of birds, fish, rodents, etc. that a druid could turn into. I think that's a great solution that still gives the druid the awesome ability that was intended without the oversight of near infinite HP.