I sympathize with your basic point. It can be a real pain when you are looking at something that seems like an obvious mistake, where you can clearly point out one or more alternative design choices that could have eliminated it in a way that almost everyone would have considered superior, and people are saying, "if you don't like it, change it", "if you were more worried about role-playing than roll-playing it wouldn't bother you", etc.
It's basically saying, "I don't see the problem that you are seeing. Therefore it isn't there."
While there has been some of that on this thread, that isn't how all of us feel about it. In my case for instance, I think that:
a) The results of the design were probably intentional.
but...
b) Designing it in such a way that saving throws and/or some alternative kept up with Save DCs better would have been more in harmony with how other editions of D&D have done things.
but...
c) Even though it stands out as an non-cohesive design element, I actually like it this way better as an objective system.
Thank you for your well-reasoned reply. It deserves (finally!) a continued discussion:
Which is simple really. I (choose to) believe that the designers saw this eventuality, but decided that KISS had precedence.
In other words, yes, that they themselves understand that given everything else the same, a game without "roll 22 on d20" would have been objectively superior.
(And now, for the first time, let our discussion move on beyond this basic fact!)
But that in order to accomplish this feat they would have had to muddle the ability bonus / proficiency bonus / saving throw with added complexity, and that they did not want that. After all, all these decisions happened before 5th edition was the success it is today.
One aspect of designing a (great) game is to recognize what elements of said game is encountered first. And to realize that some elements will only be encountered (by a significant part of the customer base) way after the crucial point where the public decides your game is a success or failure.
Having rules like "1-4 is always failure and 17-20 is always success", or putting a lower floor on your save bonus at high levels (or something similar) would have been beneficial for high-level play, and would have made play at that tier objectively better, but would also risk be percieved as a factor contributing to a general feeling (when the game is first used) of
complexity.
And that the 5E designers desperately needed a win (remember, in 2013, the scenario where Pathfinder becomes the king of D&D was very plausible; the idea may seem ridiculous now, of course, since like all d20 rulesets it appears horribly overwrought and catastrophically over-complicated), and ruthlessly pruned away at rules that weren't considered to sell the game.
At
this time, however, I think a memo to all official monster designers that basically says "keep away from save DCs above 20" (unless the effect of a failed save is something minor, like the strength saves to avoid falling prone previously discussed) would do the trick. No actual rule would then have to be changed.
(This goes for player characters too. But we have already established that if you hand out all the items in the DMG to your PCs, you will only have yourself to blame.)
As for the madness rules, I find them to be... wonky. On one hand, they appear to be designed for a gradual descent into indefinite madness (with the accumulating madness points)... on the second hand, their effects are incredibly blunt (and TBH, overpowered)... but on the third hand, the DMG is open to the possibility that a mere Lesser Restoration washes away everything.
Basically it makes no sense, if viewed as a complete and finished rules package. If viewed as a box of tools and components, OTOH, it has its uses. Shame noone told the OotA designers that, though - they treat the DMG Madness chapter as any other rule, one that is self-contained, complete and internally consistent. But you would never reach more than a single Madness point once the party gets access to Lesser Restoration if that's all you need! (Which I believe is how you must run the game in a formal environment like Adventurer's League, where you can't have table rules and rules interpretations)
And so the verdict on this entire business with Madness and the Demon Lords can only be one word:
unfinished.
Regards,
Zapp