At the very least, you're ignoring FRA, which had updated 2e specialty priests right in the hardback. And as the setting grew, it became increasingly referential to the point that rules for running one class could require 5-7 books, especially for specialty priests with unique spells and spheres from multiple sources.
I didn't ignore it, I just don't own that one.
Looking at the wikipedia page for that:
https://en.wikipedia.org/wiki/Forgotten_Realms_Adventures
It looks less like a campaign setting product and more an edition update of the 1e product. It's the post-Time of Troubles book. It doesn't look like it has much world lore at all. It's pretty comparable to
Sword Coast Adventurer's Guide really...
(Also, all that content didn't exist in 1e. So was the Realms missing "all its flavour" prior to this book?)
So, really, they had the 1987 boxed set and then this in 1990 and that was what they expected you to use for 2nd Edition. And that worked just fine.
It doesn't look like they did an actual update of the Realms again until the 1993 boxed set, four years after the launch of 2nd Edition.
And it was a jerk move. So much so, WotC promised 3e would only ever need the PHB/DMG/MM to use a supplement. (A promise that died in late 3.5).
We did only ever *need* those three. Everything else was pretty optional.
Don't recall that one, but that's a "campaign promise" if I ever heard one. A "no new taxes" or "make the trains run on time".
(And, really, you did only *need* those three. They had a wealth of accessories - for 2e, 3.0, and 3.5 - but they were optional.)
Now, if you're talking about coverting say, FRA to 5e, I agree; the fluff is (mostly) agnositic and most of the rules are supplanted by better 5e rules (such as subraces and domains) but my point was that FR in 2e required a myriad of books unless you chose to ignore a lot of the things that made Realms "The Realms" vs. a Greyhawk clone with different names and maps.
But if you ask people what the difference is between Greyhawk and the Realms, the readily given answer isn't "speciality priests", "spellfire", or kits. It's heroic high fantasy with good versus evil opposed to morally grey Swords & Sorcerery. An ideal Realms campaign is very different in tone and flavour than an ideal Greyhawk game, even if both are using the exact same rules.
(I'm uncertain how the Realms is a "Greyhawk clone" anyway, when the creation of the Realms predated Greyhawk...)
So much of the above can be supported through flavour. You don't *need* separate rules for each priesthood or every small subrace of elves and dwarves.
There was a wealth of lore in 2e (and 3e), but that was because of the heavy release schedule. Because they wanted to release so many books they expanded and added options onto the Realms. The material wasn't needed by the Realms, it was needed by the publisher who wanted to sell books. It's the definition of "tacked on". And if that material is now "essential" to the Realms then it's impossible to fully update the Realms, since there's no way WotC is going to release the 30-odd books "required".
The thing is, if they were going to release a big 320-page hardcover setting book for the Realms (likely with a MSRP of $60) then devoting 75-pages to player content - a quarter of the damn book - means 75-pages that isn't being used to describe people and places of the Realms. It's less actual setting and detail in favour of some new subclasses and subraces. Unless that content is super essential and unique to the Realms (spellscars, spellfire, shadow weave, Red Wizards) then it's pretty wasted.
But, paradoxically, if there is not hard 5th Edition mechanics, then that book can just be replaced with the FR Wiki or the exiting versions of the campaign setting.