Ilbranteloth
Explorer
I played AD&D as my preferred edition for 30 years (skipped 3e and 4e). And in my gut feeling (and experience)? There were two type of uses for the XP tables:
- standard play, from level 1 to low teens
- Monty haul. you either got to the high teens super fast, or you just said screw it and started at level 20 and ended up at level 36 after a couple weeks
So from a designer standpoint, if I see the above is true, it's entirely reasonable for me to say "We're just gonna ignore Monty Haul and focus our design on a standard level of play in this new edition, so level 20 is more than enough of a level cap."
We had a different approach.
Up through the teens (although it's pretty rare for any characters to even reach 10th level in my campaigns) - PCs.
Anything above that? NPCs.
I believe it was Ed Greenwood who mentioned that the high level of the NPCs in the Gray Box were (paraphrasing), "specifically so they were higher than any PC and would always be a challenge."
I seem to recall that in Gygax's original design concept, that any spells over 6th level were also for NPCs so there were magical effects beyond the capability of the PCs.
In 5e I think the level 20 cap is essential for their concept of bounded accuracy. Not because of hit points, attack modifiers, etc., but because of special abilities and the "capstone" design approach. An approach that, not surprisingly, I'm not fond of. You have characters (apparently every one of a given class again) sprouting wings and such. Continuing that upward trend would become absurd (if it isn't already). Having said that, the boon approach works well for designing those extra-powerful NPCs.