Right. And a wizard or cleric of equivalent level can gate in celestial great wyrms or solars, use time stop and set up 2-5 deadly combat spells, inflict 24d6 damage to others at 1200' range, or just teleport away from any hazardous situation.
20th-level characters are SUPPOSED to be that mighty; how else could you rate ancient dragons at CR 20 and expect anything to survive an encounter with them?