I'd be more sympathetic to the "FR isn't the default setting" position, if it wasn't for the the stuff on pages 30 & 31 of the Player's Handbook. Most of that text is pretty useless if you aren't running a FR game.
That's a pretty minor setting-specific detail for the whole book, and they introduce it right away by saying, "In the Forgotten Realms, nine human ethnic groups are widely recognized ... These groups, and the typical names of their members, can be used as inspiration no matter which world your human is in." In other words, they present the material as being one possibility, but not rote for the game as a whole. Compare to 4e, which established a uniform default pantheon, and many other default (though still very vague), setting expectations in its Core Rulebooks. The "points of light" setting was very open-ended and had maybe a few set locations, but the core rulebooks still presented a lot of "default" assumptions about the various species and their backstories, cosmology (shadowfell, feywild), pantheon, and probably other things as well. When I ran a homebrew campaign for 4e, I couldn't help but find myself adjusting the setting (as I originally envisioned it) to better match the expectations of the 4e core rulebooks.
5e is much more upfront about presenting several alternate approaches to pantheons, character species and background, as well as subspecies, with possible translations of those subspecies for different published settings. It's subtle, but it really does encourage the idea that D&D can be played in any world that
you want to play it in.
And when I did create a character for Adventurer's League, it was very helpful to have those 2 pages, as I know nothing whatsoever about the Forgotten Realms, but I was able to quickly pick a human culture for my character with nothing more than my Player's Handbook.