Nothing. The old Ravenloft is still there, and still has its die-hard fans. What they destroyed is the idea of what Ravenloft is in the minds of anyone who might be exposed to it. Now, this new version is what they see.
You're going to need some evidence for this...
Every time a franchise is changed in a way that is incompatible with the original, you are forcing a choice from old fans, and completely supplanting the original ideas for the new fans, who have no reason to look elsewhere when the shiny heavily-marketed version is thrust in their faces. Add to that my sincere anticipation for VRG until I actually got to see it, and that's where I stand.
...because this is blatantly wrong, for several reasons.
First, people are
not just looking at the book and that's it. This isn't like the old days when you were lucky to get an article for Ravenloft in Dragon Mag and were otherwise stuck. People are going on forums and saying things like "hey, where can I find more information on [domain]. It seems kinda cool but I want more details." And then everyone else on the forum is going to tell them what's in the original material. I just googled "Ravenloft Mordent" and got many links to old and new sites, articles, and blogs, several youtube posts. R/ravenloft has over 6,000 members (the Forgotten Realms sub has only about three times that number). One of the most recent posts there is someone who wants to do a combined and updated version of Van Richten's Guide to Ghosts and Children of the Night: Ghosts. Another person is wondering why Nathan Timothy was fired from being a darklord, and neither Verbrek nor Arkandale were mentioned in VGR. A third person who says they are still learning about the setting is asking what the best RL adventures are. Another person wants to know about Rokushiima Taiyou, and specifically what the Fraternity of Shadows fansite had to say about it.
The old information is there and is
still accessed. Even if they aren't buying the old books, people are still reading, using, and adapting the old information.
Secondly, nobody is forcing a choice, and they're most especially not forcing a binary choice. It's not "old books"
or "new books." It's old
and new, in whatever combination you want, both at the same time. Maybe you prefer Darkon to be a single country ruled by Azalin and his secret police rather than a country that's been broken up and ruled by a few pretenders to the throne, but at the same time prefer zombie apocalypse Falkovnia to Naziland Falkovnia. Easy-peasy. I myself will probably use a slightly modified Dread Metrol instead of either version of Falkovnia. From what I've seen on r/ravenloft, I think most people are going for this hybrid approach--or are taking the separated domains as permission to make their
own domains.
And thirdly, how did they destroy what Ravenloft is? It's a land where beings of unforgivable evil are locked in a prison of their own making--one that they
could escape if they could truly look into themselves, understand why what they are doing is wrong, and then change, but they never will be able to because of their own hubris. It's
exactly the same as what it always was. The horror is still there. Only a few details are different, to be more inclusive and to remove old, bigoted content. The end.