I'm not sure I would characterize 1st Edition AD&D as "less complex". In many ways the evolution of the D&D ruleset has consisted of removing unnecessary complexity to make the game easier to manage. For example: a standardized simple formula for computing attack rolls instead of pages and pages of "to hit" tables (I remember when bootleg copies of the tables in the 1st Edition DMG were passed around in my school because the book had just come out and not everyone had it yet); standardized experience points to gain each level instead of class-specific; a more standardized pattern to class progressions instead of a hodgepodge of ad hoc rules; standardized mechanics like skills and feats instead of class-specific ones.

Without going back and checking, wasn't PF a fork of the open core of D&D/3.5e?

> Perhaps more, because D&D 5 was in some ways (though not all) an explicit attempt to go back to earlier, less complex editions of D&D.

D&D/4e and 5e are pretty clearly the most simple versions outside of the simplified versions after OD&D and in parallel with AD&D (B/X, BECMI, Black Box, Rules Cyclopedia), and in many ways both newer versions are simpler so long as you only consider similar similar scope, though they have broader (but not a superset, because neither has domain management or divine ascension as a first-party subsystem, while BECMI has both and the Cyclopedia retains BECMI’s coverage of the former) scope.

5e seemed to be more an effort to restore flavor elements many perceived as lost from the very complex AD&D/1e through D&D/3e than to revert to simplicity lost (since the opposite was true) in 4e.

Yes, this is why it was often referred to (at the time at least) as 3.75. Mechanically, if you knew 3/3.5 you could almost jump in without looking at any rules in Pathfinder. It did change things around spells and classes (having its own being the biggest issue more than changes to the standard/common ones with 3/3.5).

I see someone's forgotten about weapon speed factors, weights measured in gold pieces, modifiers based on opponent's armor class, nonweapon proficiencies with different numbers of slots required, ranged being measured differently based on environment, round vs turn distinctions, and any number of other things.

It's just that that one system had a lot of moving parts. Every point of AC had to come from somewhere, everything had to be typed, and there's absolutely a lot of logic behind it, but instead of just AC, you've got touch AC (without the armor bonus), flat-footed AC (without the Dex bonus), etc.

They have different kinds of complexity. One is a patchwork of countless little subsystems that you can never all remember, and the other has one really big system that does everything.

Though the real complexity in D&D 3 is in the builds. A whole industry sprung up around figuring out new broken builds, what classes to dip for what bonus, etc. I think the easy multiclassing was a mistake.

(Despite having played RPGs since 1984, D&D 5 was the first D&D that I ever bought. I was mostly playing other systems.)