Over the past few days I've seen a bunch of different suggestions as to what Blizz should do regarding these legacy type servers. Ranging from opening a realm of their own (and possibly even incorporating Nost's database and source code) to offering licensing options to 3rd parties so they can run their own servers.
While these suggestions seem great, IMO Blizz won't pursue any of them for a multitude of reasons. I think one of the main reasons for that is Blizz wants WoW to remain a Blizzard product. They want to be the only ones offering the game, in order to maximize the exposure and profits for the company.
In my opinion, the best option for Blizzard to not only maximize realistic profits as well as prolong the life of their game entirely would be to open servers that completely restart the game. Basically just start everything over, start at beginning and release the content on a similar timeline to nost's. Hopefully what this would do is allow people to play these rolled back servers, and enjoy the game for what it was in the past; while concurrently solving the problem of the servers going stale. To me it seems that Blizz is mainly concerned with the long term results for these type of servers, that once people reach the end of the content for that expansion they'll be less motivated to play that server. So by basically restarting the game, these servers can essentially live as long as the current retail servers.
The truth is retail WoW is dying out, its apparent to everyone. Since WotLK not only has the number of subscribers fell, but the overall quality of the game. Opening these "restart" servers could allow the game to be played on blizzard servers long after the retail game's ultimate demise, and preserve the essence of WoW for future players.
One issue I could see them having for this idea is the subscription factor. They are afraid that people do in fact want to play these expansions, they just don't want to pay a subscription fee for them. My solution for this would be to simply tack a price onto the game each time a expansion is released on the realm. For instance when the server is launched, blizz could charge 60$ (the average price for most games) and allow free online play, then rinse and repeat for each expansion. It may seem unfeasible from the start, but in my opinion people would rather pay more upfront and not have to worry about a subscription or recurring fee.
In conclusion I think this would work out as the best possible option when it comes to rollback or legacy servers. Opening these "restart" realms would not only attract people to play initially, but the release of the content as well as expansions would drive the motivation to play. Blizzard could potentially double the life of their game, and allow those who didn't have the chance to play the old content that option.