That doesn't strike me as being the case. A 20th-level spellcaster who starts with an 18 in their spellcasting stat, uses a +6 enhancement bonus item to boost it, has received a +5 inherent bonus to it, is putting all five of their ability increases into it, and is casting a 9th-level spell which they have Greater Spell Focus in, will have a save DC of 33.
A 20th-level character using their good save against that spell will have a +12 from the base save value. If we presume that they have a cloak of resistance +5 (only 25,000 gp for a +5 resistance bonus to all saves), and we conservatively estimate that their relevant stat for the save is 20 (for another +5 bonus), that's a total of +22 to the save, meaning that they succeed on an 11 or above (i.e. a 50% chance of success).
Now obviously, you can tweak those numbers in a lot of ways for a lot of different circumstances, but there has to be a pretty huge disparity to have an instance where you can only succeed on a natural 20 on your good save.