A weird thing about monsters

[Skrum]

Are you saying it should never drop below 25% WITHOUT external help/magic items? Or are we assuming a least a minimal level of party support/magic items (Bless, Enhanced Ability, X of Protection, etc.)?

I think each class should, entirely within their own power and without spending any particular build resource (ASI or the like) should be in that range against level-appropriate threats. Maybe certain extremely powerful enemies should push that a bit higher - like a dragon's breath weapon or a lich's spells should perhaps rightfully have high DC's. But each class should feel like a competent adventurer.

If a whole party decides they want to stack saves (the cleric is always concentrating on bless, everyone stands in the paladin's aura, and the artificer always saves their reaction for Flash of Genius), well obviously they're not gonna fail many saves. But even in that case, I think those abilities are ideally tuned such that 75% is something of a soft ceiling.

What I think the fix here is? All characters are proficient in all saves. And then save-boosting abilities (flash of genius, aura of protection) are nerfed. That way even if saves are stacked, the upper bound still doesn't reach "just don't roll a 1" level.

While I agree it FEELS wrong for a Barb to be vulnerable to fear, it's important to remember that Fear effects generally aren't a "This is a scary monster, so I'm Frightened" thing. It's generally a "there's a magical effect attempting to trigger a specific emotional response" thing. The level 17 Barb isn't actually afraid of the Quasit, the Quasit is triggering a magical fear response. And if "weak against mind-effecting magic" is part of your class, it makes sense that even the low-level versions might still be able to effect you, and you have almost no chance (without external support) against a boss monster that target that weakness.

Levelling up doesn't always mean we outgrowing our weaknesses. We still expect an ice elemental to be vulnerable to fire, no matter how many hit dice it has.

True enough, but I can tell you that in the moment, failing that save to a fear effect and not getting to move or having to drop your weapon and run away, like that just sucks. It's about the opposite of heroic, and it's frustrating to get hit like that.