Kinder reader Inflection wanted
A valid point. Losing isn't always about not dying. Even if you end up with a PC that has 1000+HP and AC39, it doesn't mean anything if they don't do enough damage to kill the enemies, are not cleaver enough to solved the mystery, or don't work well enough as a team to manage the mutilple tasks they need to complete before time runs out.Best way is to ignore them entirely or Target saves. A massive pool of HP doesn't help the wizard or the commoners caught in the middle and failing a save can be more painful than any amount of HP damage.
Besides that at level 20, the gloves are pretty much off for the GM., even if the GM does TPK the party they had 20 levels and not much more to gain. Instead you wipe the group and the next campaign can be about weakening the great and powerful enemy that did it by achieving 20 tasks that each take way its power, reduce its minions, kills its allies, and break down its defenses. Sounds fun to me and your last party is epic lore for the new one. : )