You've got some good points in how you're characterizing the editions, but I don't think I'd really agree with this one. There weren't many years to accumulate additions to 3e and make them official. Plus, too many of the changes weren't additions - they were changes in approach and standardization that kind of repudiated some of the additions (like the other hit point adding feats of the splatbooks beyond toughness). There were some things that were definitely fixes to under-performers or problems such as the ranger, bard, harm spells, and reducing the front-loading of character abilities exploited in multiclassing. But there were a lot of changes made to further regularize ranges, areas of effect, and durations - some of which might be based on the experience of stat-boosters and metamagic becoming dominating strategies, but others of which indicate an increasing focus on "the encounter" as the most important unit of concern when designing/balancing the game rather than adventures or adventuring days - and that shows the evolution toward 4e's mode of thinking.
The other major proud nail of 3.5e, if you ask me, is the shift to weapon/armor sizes being an issue. It's the only D&D edition that really does it. I can't decide if it's a push to take the way 3.0's effort to increase the rationalization of the game through game mastery to a higher level or someone's quirky take on "the way things should work, hang the game's history".
I can, largely, agree... but.
Between July of 2000 and July of 2003 there were:
8 player-focused splatbooks (Savage Species was the last of them)
4 monster-focused splatbooks
2 Optional Rulesets (Epic Level and Vile Darkness)
2 Settings (Oriental Adventures, Forgotten Realms)
7 Forgotten Realms splatbooks (with player options)
3 Environment/Situation splatbooks
1 Spell and Item splatbook
1 Starter Set
That's 28 books. 9 books a year, or 1 book every month and a half. And that's the OFFICIAL books.
The first 3 years of 3e were INSANE in the 3rd Party space as everyone jumped on the OGL bandwagon and released great gobs of stuff. Including ways to streamline gameplay or shift things around... some of which became part of 3.5e since the OGL is a two way street.
Don't get me wrong. There was definitely cleanup of material. But in 3 years there were 28 official books to 5e's 36 across 11 years. And that's including the tiny releases like the Plane Shift series and One Grung Above. Take those 7 off and it's 28 vs 29, 3 years versus 11 years.
That said, you're right that 3e definitely needed some reigning in as part of the reason 3.5e came about so very, VERY, quickly...
And then they went on to do another:
19 player-focused splatbooks
11 monster-focused splatbooks
7 Optional Ruleset books
6 Environment/Situation splatbooks
1 Campaign Setting (Eberron)
16 Eberron splatbooks
12 Forgotten Realms splatbooks
3 Spell and Item splatbooks
3 Starter Sets
And a partridge in a pear treeeeeee... 78 more books in the 5 years before 4e.
And the first three 3.5 player-splats replaced and added on to the previous player splats. Complete Arcane replaced and expanded on Tome and Blood, Complete Warrior replaced and expanded on Sword and Fist, and Complete Divine replaced and expanded on Defenders of the Faith -AND- Masters of the Wild, before the Complete Adventurer replaced Song and Silence. And hey, Expanded Psionics replaced the Psionics Handbook the same year. FUN.
Which goes back to the core idea of "Taking old ideas and refining them to work in the new system while also adding new stuff)... even though the books being replaced were only 2-3 years old.