The essence of D&D has usually been a group of wanderers risking there lives in a dangerous world. At about tenth level or so, they become among the most dangerous things in the world, and the tension dies out. If the PCs feel like the biggest badasses on the planet, then that's a very different game.

My vague idea is that when the party sees a dragon and does not feel threatened, and does not consider fleeing, and does not come up with a careful plan, but just yawns and attacks from the front, the campaign is over.

[I get that epic adventures exist. They are a very different kind of game.]

Quote Originally Posted by Morty View Post
On top of that, the franchise never really had a good idea of what higher levels (as in, past 10) are even meant to look like. Below level 10 or so, it mostly knows what the place of the PCs in the world is meant to be, even if it can't quite handle its power level. But when you go beyond that, the PCs' abilities make the world around them increasingly irrelevant, and the rules and setting don't really know how to portray that.
There's a lot of understanding here, but it's not true that the franchise never knew what high levels should be. It's closer to say that what high levels should be didn't really appeal to people who'd played their way up to it.

In original D&D, at about 10th level, PCs were supposed to stop being wanderers, clear out a fiefdom in the unconquered wilderness, build a keep and an army, and now the adventures come to you, in the form of invaders.

This worked well for the original players, who were miniatures wargamers first. But as soon as D&D was published, there were lots of new players who were in it for the adventures. They didn't want to settle down, and they didn't want to play an army. They wanted to play Gore-tex the Dragon-Slayer.

Meanwhile, their PCs had outgrown most of the monsters and encounters, which were designed to get them to tenth level.

The game was not designed for higher level wanderers, but higher level PCs kept wandering.