Pre-3e mechanics vs d20 system mechanics

[Neonchameleon]

I'm going to float a hypothesis here. Almost all mature games that give any attempt at being even vaguely along the process-sim spectrum, that have survived playtesting, and that don't have designers deliberately keeping them rules light or rules medium-light end up overall at about the same "rules medium-heavy" level of complexity. This is because people aren't that different, and there's only a certain level of rules complexity that people will find fun and much more than that and the game will collapse under its own weight..

Unified mechanics are simpler, period. And addition is easier than subtraction. But it was this easiness that gave 3.X the slack to add so many modifiers and build the complexity into the 3.0 and 3.5 we all know. 4e of course added the complexity to the characters with the power structure and lots of debuffs (which had the advantage of letting it be stripped back for simpler characters so although 4e can be incredibly complex and fiddly it doesn't have to be). 5e made a deliberate effort to take AD&D rules on the cleaner d20 structure.

 

[Tony Vargas]

Unified mechanics are simpler, period. And addition is easier than subtraction. But it was this easiness that gave 3.X the slack to add so many modifiers and build the complexity into the 3.0 and 3.5 we all know.

True. d20 did consolidate baroque dice mechanics down to d20+bonuses vs DC. Then it added 18 named modifiers to keep track of. And, 40 conditions...

4e of course added the complexity to the characters with the power structure and lots of debuffs

... 4e actually consolidated those 40 conditions down to 18 (5e further, to 15). And the power structure and class advancement was also a further consolidation (3e had put all classes on the same exp, HD & save progressions; 4e also put them on the same AEDU and "BAB" progressions).
So that was all simpler, really - confusing & unfamiliar if you were accustomed to earlier eds, but less complex.
But, following your hypothesis, like 3e, it took that slack and wrapped it up in potentially enormous build complexity, just a sheer volume of options. (Most egregiously, thousands of pointless 'chaff' feats.)

(which had the advantage of letting it be stripped back for simpler characters so although 4e can be incredibly complex and fiddly it doesn't have to be). 5e made a deliberate effort to take AD&D rules on the cleaner d20 structure.

Even with d20 consolidation of some of it's more pointless sub-systems, AD&D is still a pretty needlessly complex structure to slave a modern design too, and it shows in 5e's version of it. Or, rather, it /doesn't/ show if you've internalized all that AD&D weirdness. The only things that look weird and needlessly complicated to jaded eyes are the neo-Vancian prep-then-cast-spontaneously, because, well, it is more complex (though simultaneously more versatile), but mainly because it's just /different/ from traditional Vancian... but, with spells still in the familiar 9 levels that almost but not quite map to level/2, I guess it's OK...
... of course, 5e did make up it's loss of complexity slack somewhere: by doing away with all those 3e & 4e options & build complexities.