To add to this, to everyone asking "Why look backwards?" (copypasting my answer from another thread from here on out)
Well, for one, posterity. World of Warcraft as it existed in 2005 is impossible to obtain and play legally right now. The environments (barring instanced maps), dungeons (again, barring the maps, but a few of these were also changed/condensed), class designs, storylines, and many items are all currently lost to history. In essence, there's an entire MMORPG, one that kickstarted the genre's shift into the mainstream rather than being the butt of the 'sweaty, greasy EverQuest nerd' stereotype once celebrities began coming out as WoW players coupled with Blizzard's aggressive and successful marketing that's lost to the aether.
So a legacy server would accomplish a fairly important goal in preserving WoW in a form that currently doesn't exist (the expansions are less subject to this arguing point, as they're largely unchanged from release other than character design like talents and abilities). Given WoW's place in video game history, that's a fairly big deal. That it would also appeal to a demographic that may not currently be giving Blizz money is just the financial carrot-on-a-stick that could get the team to justify it to the suits.
I wouldn't spend a lot of time on it, myself, but I can see the value in preserving history rather than just constantly forging onward without any time taken to study the past. Or just go back and play an old game without having to step into legal grey areas to do so; again, classic WoW is an outlier here along with other dead MMORPGs as I can still buy an Atari 2600 and play the SwordQuest games, or grab a Sega Genesis and play Phantasy Star II (which is historic as one of the first RPGs to feature a major party member's permanent plot death midway into the game).
Unless the Library of Congress starts hosting servers to run MMOs that had a huge impact on the genre and are no longer available to play, right now the only choices are to play them on private servers, which involves making a few moral and ethical compromises, or continue lobbying Blizzard to put up a server for posterity.
There's also that contingent who just hate the modern game and would rather revel in nostalgia, and that's just as valid given, again, the game is completely inaccessible through legal means right now and those players bought a game with the intent to play it for a pretty long time, given the nature of an MMO.