I'm gonna respond to your points a little bit out of order, because it's more expedient for me to set up the topics I'm going to talk about that way.

Yes, you can just get rid of long-lived species if you like. You can also modify the world to match the fact near-immortals exist and I don’t think it’s that hard. It’s your decision, ultimately, but there’s a lot of ways to solve it.

I agree with this. One of my favorite settings, World of Darkness (specifically the Vampire versions of OWoD), is completely defined by the fact that immortal beings are present in the world, and their machinations dominate everything about it. And one of my favorite DnD-like settings, Arcanum, deals pretty heavily with how humanity interacts with longer lived races, and major parts of its backstory are defined by the actions of the longer-lived elves and dwarves.

The thing is neither of my DM friends that I mentioned in my original comment wanted to deal with the elven illuminati and they didn't want to make elves senile. Their world was also not very Tolkien-esque, so "elves spend most of their lives doing nothing" didn't fit the vibe they they were going for.

And a fourth point, Elves may not care/notice at all. If the Elves are insular and live in the woods they’re extremely unlikely to bother remembering the Human king, after all he only lives like a scant 100 years at most so why even know his name?

This is what my friends originally did with elves before they got rid of them, but they had another issue. Very often they would like to end one campaign, do a 100 year timeskip, then start another one in the same setting. If a player character was an elf that meant that they should still be around, and in good health. This was problematic because these campaigns were generally fairly low level, and they didn't want to have a high level NPC running around.

I'm sure there are ways to deal with that too, but with all of the other issues elves created I gather they decided it wasn't worth the effort.

Depending on your depiction of Elves the effect is the same (provided the group is Elves) because they’re often predicted as just slower/more leisurely in their approach to life (although I’m not sure what you mean by the statement in the first place, because something being older than you intended doesn’t sound like an actual problem).

So, I should have made this more clear in my original comment, but there are no non-human races in the world that I am building (it's not even for a DnD campaign). That anecdote was just about me finding out the hard way how long 15,000 years is.

As to why it's a problem if the knowledge keepers ended up being older than I had originally intended it's because it's really hard to keep a cohesive organization with the same goal around for thousands of years. The core of the Jewish religion is probably the most successful at this, and it's extremely impressive. Some native American and Australian Aboriginal cultures kept accurate oral records for even longer, but that was within a completely different social context.

So I didn't want my fictional group to do a 2x or 3x better job than real life people. Likewise I didn't want massive empires lasting way longer than real life empires.

I can go into more detail about my world if you're interested, but I didn't want to make this comment any longer lol