I agree that "core 3 is a 3e thing". It always bugs me when people claim 4e trying to make more things core was somehow a scam. EVERY edition did this EXCEPT 3e. Only 3e made that distinction (and it was for licensing reasons), but somehow 4e going back to the way it was done prior to 3e was viewed as some sort of scam on people...even though even Pathfinder is no longer following that 3e conceit by publishing adventure paths that reference more than the core books.
Actually, early on TSR always assumed people only have the Core 3 books.
This was actually one of the reasons for 2nd Edition. They liked some of the new rules they came up with for
Oriental Adventures and other books but could not expand on them because the books were not assumed. But by making a new version of the game they could slip proficiencies into the game.
2nd Edition assumed a few more books.
Legends & Lore and
Tome of Magic were occasionally assumed. But other than that, the game assumed little. 2e also had fewer big setting neutral hardcovers.
The "everything is core" idea had some benefits, but also had some problems. Assuming campaign settings and side accessories were core was questionable. And treating the magazines as an equally valid source was iffy, especially since the content was not getting the same development passes (they claimed that they were going to increase the time that went into developing
Dragon content on two separate occasions). It was great that new classes and options did get support in later books
Personally, I like Pathfinder's approach. The big hardcover books are assumed, but everything else is not. And even then
Ultimate Campaign and
Mythic Adventures are pretty close to optional. But when publishing APs or Pathfinder Society adventures they tend to assume the big published hardcovers are available to reference. And even then, not every monster is assumed to be part of the world (especially the new humanoid races).