Mecheon
Sacabambaspis
That's a homage, a fun little thing for repeat readers. It isn't an overall metaplot. This hardly invalidates anything I have said. The age of the big ol' setting metaplot died years ago. There's a timeline if you want to play them going into each other, sure, but is this some big setting wide metaplot that all products adhere to? No. They've long given up on that and just "Whatever you want to happen, it happens. Want this in another setting? Here's some ideas of where to put it. Have fun!"An NPC from Princes of the Apocalypse appears in Dragon Heist trying to redeem herself for example. So your point is invalidated in this example.
I do not see the need to be throwing more more Forgotten Realms stuff over and over when no other setting is getting that sort of treatment, and especially when other settings are lacking anything on them to even mechanically come into 5E.
And additionally if some future FR book comes out, unless it has something that's mechanically useful to me, I can 110% confirm I will consider it a useless product and not buy it if it is just setting and lore information