Something I really don't understand about this discussion is how any FR fan could possibly complain about changes to canon and retcons. Good grief, every D&D setting has been retconned extensively over the years. Every time we got a new edition, the entire setting got retconned.
Just off the top of my head. In 1e, all FR clerics were just that, clerics. Then, in 2e, you have specialty priests, which used a different xp table, different weapons and armour and completely different spell lists. My 2e Priest of Kossuth from Faiths and Avatars had access to all wizard fire spells as cleric spells, could use swords, and, by 5th level, could summon a fire elemental once a ten-day. 3e rolls around, and all that gets entirely retconned yet again as clerics are now shoe-horned into 3e's class structure. My Priest of Kossuth was now impossible to make in 3e - or at least extremely difficult and would take about double digit levels to re-create. 4e rolls around and redoes everything once again, and now 5e has restructured things for a fifth time.
How can you possibly complain about setting ret-cons in a setting that is so full of holes as FR? Tieflings and Genasi? Didn't exist until 1996 AT ALL. Yet are retconned into the setting in 3e as always being there AND being relatively common to find. Certainly not all that rare.
What's the problem with bringing, say, Tharizudun from Greyhawk in FR when we've already brought stuff like that from Planescape? A complete set of PC races is far, FAR more intrusive than a single big arsed boss monster that will only come into very high level games.
Heck, Sorcerers didn't exist in the setting until 3e because, well, there were no sorcs in D&D until 1999. Yet, suddenly, sorcerers everywhere in FR. Poof, instant retcon.
The problem, as I see it, for "why don't you just ignore the lore" is that those who are telling me to ignore the lore are pretty bloody picky and choosey about which lore to get uptight about. Never mind the fact that in order to ignore the lore, I still have to learn it in the first place. It's a lot more difficult to start rewriting canon if you have no idea what came before and, as is evidenced by this thread, people get uptight when you start doing that. So, I still have to absorb tens of thousands of pages of lore, scattered across hundreds of publications, spread across over thirty years in order to have a decent grasp on the setting that doesn't even maintain it's own continuity and never has.