Like a Facebook relationship: Its complicated.
D&D should have its own lore. It's not Generic Fantasy RPG d20, it is its own thing. I am perfectly fine with their being specific info on D&D elves, dragons, deities, and such. A D&D game should be unique. I'm also fine with the Multiverse they've implemented as the catch-all for the various setting/genre books they are calling campaign guides.
That said, every edition of D&D brings new rules and new assumptions, and settings should adapt to those edition's assumptions if they are going to support the game. I would be very cross if (for example) a 5e Mystara setting didn't allow dwarves to be wizards because "they weren't wizards in Basic". That is lore that needs to adapt to the 5e paradigm of any class, any race. Similarly, I'm fine with any WotC setting introducing tieflings and dragonborn in them even if they didn't have them prior to 4e because that's what's in the PHB. Settings serve the game, not vice-versa.
In specific examples like Ravenloft and Dark Sun, extra concern is warranted as both settings are very genre dependent and what is/isn't acceptable in those genres have changed. It doesn't bother me that these two settings are more spiritual successors than direct continuations. I imagine Dragonlance and Greyhawk would get similar (if not as radical) reboots to their classic eras with an eye for modern audiences. While I may not agree with every change, I agree that these products need to evolve, innovate, and meet the demands of newer audiences. If something in the lore has to shift to accommodate, I'm willing to accept.