If you have a low stat at low level, lack of proficiency to saves makes only a small difference - even if you were proficient, you're going to fail quite a bit. At high level, they do, indeed, make the low stat a more profound liability.
It's OK for high level character to have a liability. With access to things like Indomitable and Restoration and Freedom of Movement ant whatnot, high-level characters have access to a
menagerie of effects that can undo a failed save.
A typical low-level DC is 13. Proficiency makes the difference between a -1 and a +1 for a character with an 8 stat. Save on a 12 vs save on a 14. At highest levels the save gets as high as 19. A natural 20 would be required for that 8-stat character. With proficiency, he's still worse off than he was vs a same-level threat at first, now he needs a 14 instead of a 12. Worse, but not a lot worse. Without proficiency, he'd have to boost his stat quite a bit to have any chance of making saves, making the low stat 'less viable,' indeed.
You an call those numbers a preconception if you want, I'd say they're just a reality.
The perception is that the saving throw is the only thing that matters here, and that's not accurate. Analyzing DC's in a vacuum gives you a distorted view of how the game plays at the table. For instance:
I mean, consider a multiple-save effect like Hold Person. If you need a natural 20 (and, if the NPC caster's DC is 19, it's 19 for all his spells, even the trivial little low-level ones), you're not just failing 'most of the time' - even advantage barely helps you at all. You'd be lucky to break out of a Hold Person in 10 rounds.
Hold Person is a
concentration spell. What the hell is the rest of your party doing, twiddling their thumbs while you're incapacitated? Or are they cool with you "tanking" the Concentration slot for that so that the spellcaster can't concentrate on other shenanigans?
Yes, if your 8 WIS warlock happens to get a high-level
Hold dropped on them, they won't be able to rescue themselves unless they get very lucky. That's OK. No one delves these dungeons alone. Part of what that does is adding a team dynamic to play - you have to rely on your party members when you get taken out.
Tony Vargas said:
Giving all PC classes proficiency in all saves won't make the game any worse.
Yeah, probably not. But what's it gonna add, I wonder? More successful saves, fewer moments of tension, less effective enemy spellcasters....it'll make the game significantly easier for the PC's.
Because 5e is a game whose play experience can be so dramatically different from what the ability looks like on paper, I'm suspicious when a change is proposed whose goal is to fix a thing that
seems broken. If the intent here is "I want PC's to be more powerful," I'd say, yeah, this does that perfectly. If the intent here is "I think weak saves at high levels are a design flaw," I'd say fix it only when and if it becomes a problem in play, and don't sweat it before then.