The Smurfiest Wizard Ever!
You're missing the point. The issue is a level 1 rogue and a level 20 rogue will both fail the DC 12 (NOT talking powerful, here ) CON save the same amount of the time. Especially in a feat-accessible game (85-90% of tables), it is unlikely the rogue with use an ASI on CON. Now, they might use it for Resilient, but then again there are a lot of other feats I see rogues taking first.The rogue should fail the save more often when it's against more powerful enemies. An ancient dragon or a powerful lich should be more dangerous, even to the PCs, than a wyrmling or an apprentice necromancer.
edit: again, that isn't a case of the rogue getting worse at saves. They're the same. They're just facing much more dangerous enemies. Those are two different things.
Anyway, this is despite the fact that the level 20 rogue has probably had to roll dozens, if not hundreds (well... maybe?) of CON saves during his adventures. But, he is NO better at them???
That makes no sense, whatsoever.
Now, I know the argument. Higher level PCs have more hit points (WotC's answer to everything, apparently) so the failed saves don't affect them as much. But, that is another leg of the HP bloat issue in 5E. Sure, its a "feature" of the game--a horrible one IMO.
We have a sorcerer/druid in our main game (level 15) who, naturally, lacks DEX saves. So, that DC to avoid that trap or Lightning Bolt or whatever is just as likely to fail at level 15 as he was at level 1, despite facing numerous traps, fireballs and lightning bolts, etc. during his 15 levels of adventure and danger.
Finally, back to the original claim (which is entirely accurate, BTW, since I never claimed why they are more likely to fail...):
After all, higher level PCs are more likely to fail their saving throws then at lower levels. Sad, huh?
Due to the fact they will be facing more powerful foes, AND 2/3rds of their saves are unlikely to improve in general, higher level PCs are more likely to fail their saving throws then at lower levels. I say again: sad, huh?
Our table has discussed it at length, and it makes no sense to any of us, so FWIW when we start our new campaign, the house-rules (which are not "broad" but cut "deep" into 5E mechanics) will have ALL creatures (PCs and monsters) proficient in ALL saves. PCs' selected saves will gain advantage, as will monsters with listed saves in their stat blocks. Now, don't worry, other parts of the "cutting deep" make it all work out nicely.