Lol, there is no chance for Blizzard putting up official legacy servers because:
1. This would be Blizzard admitting that their current game sucks and the previous iterations were much better;
2. It would require considerable splitting of resources to maintain the old version of the game along with mainline WoW;
3. It would discourage players from buying new expansions;
4. As evident from Old-School Runescape, not that many people would actually play on the legacy servers in the first place.
As evident from Nostalrius and countless other Vanilla servers, there is clearly plenty of demand. I can't even imagine you would need any maintenance would you? Once they publish the servers and finalize the code to where it was in Vanilla I am pretty sure that would be it, because making any refinements beyond how the game existed at the end of Vanilla would essentially make it non-Vanilla. You get the good with the bad.
I think the expansion loss thing would be countered by the influx of players who wanted to play specific era content. I mean, imagine a player who hangs out in Vanilla with his guild for a year. And then said guild moves over to TBC. Blizzard could even sell services like xfering your toon over to TBC from Vanilla.
I agree with you wholeheartedly about #1 though. I think Blizzard knows that strong interest would amount to a damning repudiation of where Blizzard has gone with WoW. I could even envision a server list where classic servers are all listed as "FULL" while the newer shit is all LOW population.. lol.
I think Blizzard is really losing out on a shit load of $$$ by denying that this demand is there. I will go even further by saying that if you want to make the argument to Blizzard, pointing out the $$$ aspect is probably not a bad way to argue the point.