Well look, Mordenkainen in 5E has shown up in two adventures, one being in Ravenloft (Curse of Strahd), and the other in Avernus (Descent into Avernus). I know the Avernus adventure starts in Baldur's Gate, but Mordenkainen isn't in that part. So I don't think Mordenkainen has been "stuffed into" FR at all.
And I was ok with Acererak being in Chult in Tomb of Annihilation. It didn't really feel like a retcon in any way, he's a planes-trotting lich who decided randomly to do his newest plan in FR.
Anyway, this comment just highlights my annoyance, as you're referring to these characters as "Refugees" as if they've been displaced or something. I'm happy to imagine them as multi-setting, powerful D&D characters that traverse many worlds on the Material Plane. It's when folks are saying "Ah, they've been absorbed and are FR material now!" which makes me roll my eyes.
EDIT: Vecna is a great example of this; he's now a prominent god in Wildemount, which is now it's own 5E setting, and yet you're spinning it as if he's suddenly going to be FR exclusive... I mean, these settings can share.