[Trigger warning] This is just my opinion and I know I could be wrong about this.
A lot of speculation has taken place over Nost's cryptic messages over the last couple of weeks. It certainly seems like they are okay with people thinking they would re-release the server if Blizzard does not announce legacy servers at Blizzcon. Either way, we all know in the back of our minds that they may just end up releasing the source code for Nost or something even less exciting (More t-shirts?).
Releasing the source code seems like it would be helpful for the Vanilla community, but I don't think that is necessarily true. Over the years many private servers have been released and many have been underwhelming or completely sucked altogether. Anyone with a coding background, a little bit of scratch, and some free time can release a private server. Only very few can release a good one and stick to it.
So what does it take to release a good and successful private server?
-Serious knowledge of code and server management
-A committed and mature team that works well together
-Enough free time to deal with bugs in a reasonable time frame while continuing to push progression
-Enough money to get things going before donations are made
-A strong commitment to the game itself and the vision of making it available to many people
I'm sure there are more qualities I am missing, but the Nostalrius developers met these qualifications and then some and are the best example of this I can think of by far.
My argument is that the need to script and debug the MaNGOS core (or whatever core people decide to use) acts as a a sort of strainer to sift through he people who are serious and the ones who are not. It separates the men from the boys. Right now, it takes years of commitment and work to get a private server up and running (if you're doing it right and aren't unemployed). This may seem like a bad thing, but without this strainer, where will the community go?
Many servers will spring up in a matter of days, trying to cash in on some easy money. The community will split across these servers, all of them having Devs who are completely unproven. None of them will have gone through the rigor and toil. Many of them will be immature and shortsighted. Many of them will drive private vanilla servers into the ground because of an inability to fix bugs, script new content, and deal with the numerous challenges that running a private server bring.
You might say that this would work itself out eventually. People will leave the servers that suck and someone will make a good one which everyone will end up at. While that's possible, it's not certain and it may take a very long time before this happens. We may never see developers committed, mature, and knowledgeable enough to create something like we had with Nostalrius or like we may have with Crestfall (I get that they are unproven at this point as well, but do seem to have many of the qualities listed above in spades).
Conclusion and TL:DR
I guess the big question is: Without the legwork of scripting, debugging, rolling back or otherwise tweaking a core, how will we know which servers are serious? How will we know which Devs are in it for the long haul? I know that many of us don't want to work our way up to 60 again just to see a server not make it.
My solution? Nost finds a team of people they trust (may involve GMs and Devs from Nost team, may not) and entrusts them with the core. The community can rally around around this server and hope for the best. Yes, this is putting a lot of faith in the people Nost entrusts it too, but better that than alternative mess IMO.
The best case scenario is, of course, Nost devs re-releasing the server themselves. This post assumes that's not the case (please let it be though!)