I actually really disliked the idea that everything had to be in the same multiverse in the first place, but I guess once you start having Plane Shift spells and Planescape as a setting and the like, someone's eventually gonna want to write a Marvel vs DC crossover. I liked Soth in RL because he fit so well, and Azalin's desire to escape at least gave him some motivation, but Kalid-ma and Thakok-An had no business being there imho, Vecna and Kas weren't much better, Hazlik didn't really add anything interesting, and most of the other lords who originated from other TSR settings could have just been Ravenloft locals. People like Lukas and Easan - is the fact that they are nominally from FR and Greyhawk actually relevant to their character or their domain? Does the setting they come from actually add anything? In most cases, no.
Sidenote - Dark Sun, with it's notion of the dwarven focus and being condemned to undeath if you died having neglected or failed it, is a far more Gothic take on conventional fantasy dwarfdom than anything in Ravenloft.
So personally while the humanocentric thing works for me from an aesthetic point of view, and I'd certainly run Ravenloft as a human-dominant setting, I just wish they'd commit one way or the other. If you're going to have non-humans in the setting, then at least use them and have towns etc where they live! Don't just blithely assume there's non-human PCs in the setting, then write a world guide that offers no explanations as to where they might come from.
If RL went all-human I'd be sorry to lose the Sithican elves, and I'd probably find an excuse to keep them, but everything else could go as far as I'm concerned (supernaturally 'altered' humans like calibans or dhampirs could stay too). If i'm playing a Ravenloft game, I'm not really interested in multiverse theory. It's a personal choice and everyone's going to have their own opinion, but i'd rather they stick as close to the gothic literary and film roots of the setting and for me it's easier to do that without having to fit in dragonborn and gnomes etc routinely living alongside humans.