Imagine an all-martial party. WotC likes to tell us that you aren't required to have any particular party composition to play the game. So we have, let's say, a Fighter, a Barbarian, a Rogue, and...I don't know, Monks are supposedly martials, right? Good enough.
Let's assume that neither Feats nor magic items are allowed- after all, these are optional elements, the game runs just fine without them!
Let's assume that each of these characters has a 10 stat somewhere. By the time the players are 9th level, the difference between their best save and their worst could be as much as 9 points (+9 vs. +0).
The party encounters, oh, say, a CR 10 Guardian Naga. It has three spells that target Wisdom (Hold Person, Bestow Curse, Geas) and one that targets Charisma (Banishment).
It's not a stretch to say that the worst Wisdom save in the party is +1, and the worst Charisma save is +0. The Naga's save DC is 16. So that's a success rate of 75-80% that one of the characters can instantly be removed from the fight (or mostly removed, in the case of that Wis save or do nothing on Bestow Curse). And while the party will likely kill the Naga quickly, as it's just one enemy, a few levels later, when you can fight the same Naga with other foes, nothing has changed about it's effectiveness against them!
This is a problem that simply gets worse as the game goes on, as enemy save DC's continue to scale, but your ability to save doesn't. And the game offers three potential solutions to this issue.
1) have an NPC spellcaster for things like Heroes' Feast, Bless, Resistance, what have you.
2) allow Resilient as a Feat so everyone can have a third proficient save.
3) add magic items to bolster bad saves.
4) have someone retire their character and reroll as a Cleric or something.
And given that 2-4 are not supposed to be required to play the game at all, this seems like there's a flaw in the game's design.