Not a terrible idea. Maybe a "seasonal" server that runs through the expansions every 6 months.
That said, I'm sure maintaining different versions of the servers would actually be a huge pain. The live ops was most likely not a solved problem at the end of an expansion.
It would take a lot of back porting security patches as well as bringing certain things forward like connecting to the current bnet.
And the customer service concerns are an interesting consideration as well - Game Masters use ever evolving tools (which evolve with the game client) to be able to more effectively help players. Changing the client will break the tools which will require new tools to be developed, new policies to be developed around those tools, new (or really, old but forgotten) types of solutions for game issues that have been obsoleted.
Just imagining this small aspect of the overall challenge makes it clear to me that this type of server just would never happen on an official basis - so many cross-linked considerations and it's really hard to make a business case for almost any of them, even if it would be a lot of fun from a player point of view. Which means that allowing a volunteer community like Nostalrius is really the ideal, outside of the reality of the law which requires protecting intellectual property in all cases in order preserve it in the cases where you really want to. Could some kind of official license be made available specifically for Nostalrius?
That said, I'm sure maintaining different versions of the servers would actually be a huge pain. The live ops was most likely not a solved problem at the end of an expansion.
It would take a lot of back porting security patches as well as bringing certain things forward like connecting to the current bnet.