It would be nice, but Blizzard has said many times, often very bluntly, that they will not do legacy servers. They famously went so far as to say – and this is a direct quote – “you think you want them, but you really don’t.” Will it happen five years from now when the game is on its deathbed? Maybe. Certainly not before then. Money aside, Blizzard is stubborn. If they let you play Old Wow to them it would mean that New Wow has failed. Like Valve, Blizzard operates under the assumption that certain design principles are immutably better than other ones. They move in one direction, toward simplicity, elegance, ease of use, whatever you want to call it. Not only has the game itself moved in all of those directions, but the simple idea of offering multiple versions of the same game would cut against their corporate philosophy. They shut down Nostalrius because it got big enough to make them look like idiots, but it could never get big enough to make them change their minds.
But as far as this is concerned:
I just really don’t think that is a concern at all. I would imagine a very large percent of the people who play on private servers would pay to play on official ones. The people on these servers are not casual players. They’re all people who previously paid to play the game, and even the best private servers have a bunch of problems that would be fixed on official servers (I played on Nostalrius for a few months from the day it launched; one of the reasons I quit is because of how many bugs and scripting issues there were). And of course for every person who plays Nostalrius, how many people exist who don’t but would be interested in checking out an official server?
Here’s the real issue on the money front, though as I’ve said I don’t think money one way or another plays into it: on live wow, there is a cash shop. You can buy mounts and other cosmetic shit. And more importantly than that, they came up with the ingenious idea of allowing players to buy game time with in game gold. I say it’s ingenious because the way it works is you buy a token off the auction house for an amount of gold that is determined by the markets on a per region basis. But those tokens don’t come directly from Blizzard; you buy them from other players, who buy them from Blizzard for the purpose of selling them for gold. The kicker here is the token, which gives you a month of game time once you buy it from the AH, costs more money to buy from blizzard than game time does. So here’s the upshot: if you play wow regularly, you are almost certainly paying for your game time with tokens, which means Blizzard is effectively seeing more money from you than they would if you were paying with money. Neither of these things would fly on legacy servers.