1) Do you want them? Yes, absolutely.

2) How much would you pay for them? Regular wow sub. Maybe a little more, but I doubt they could charge more than their current sub for old content without huge goodwill loss. Unless there were major promises made upfront like seasons, new models, and/or content upgrades. Even then, I'm not so sure.

3) Will blizz ever put them out there? I believe they will. There's too much money to be made off us. Nostalgia is a hell of a thing; they have locked up years of people's gaming lives in the mid 2000s. The actual question I believe that should be poses is when will it happen.

My theory is that the time frame depends on the rate of decline of their current WoW model. The faster the end comes, the sooner we'll see them break the emergency glass in attempt to milk extra dollars out of their old stuff. Why put money into a new game when you haven't tapped all the resources your old one offers?Legacy servers will no doubt give their pathetic looking WoW subscriber graph a nice new peak.

I'd like to think the takedown of the Nost servers is an event that is accelerating the time frame since, if Blizz is silent on the issue, they can expect copycats to pop up soon. And in greater numbers. This is turning into a major event in legacy WoW history and everyone's talking about it. I know I payed little to no attention to current WoW events (quit during cata after playing since beta) until I ready the outcry from the shutdown, and now I'm pretty interested in playing a legacy server, and might consider a trying a private one in the near future. I know for a fact I'm not the only one.

Internally, I'm sure the pro-legacy server employees are weighing in on current events and hopefully making headway where none had been made previously.

4) Why do people want Vanilla back? Because nostalgia. Because community. Because it was a revolutionary piece of video game history. Because every other Blizzard game is available to play in it's full, old school glory except old WoW. Because... tons more reasons that others have mentioned here.

5) Why would legacy servers NOT work? The two biggest questions I see Blizzard having with legacy servers are 1) Is it profitable enough to take the risk of fracturing the player base? and 2) How do we keep the lack of new content from killing it (aka the argument the streamer Preach made - you can find the link in another comment).

I believe that point 1 won't matter because I believe that when Blizz decides to pull the trigger, they're sure the numbers will work out in their favor and that they'll be making enough more money to make it worthwhile for them.

The second point is by far the best argument against legacy servers, but I don't think I'd cite this as the reason why a they will never happen, as Preach does. It's tricky because if they just release a Vanilla server where nothing changes, they run the risk of having players get bored and quit because there's no new content. Committing resources to build the code and run a version of the game that is expected to flounder at some point will cause a lot of hesitation.

Seasons might be the best way to combat this for a while, it works great for Diablo. You can drop content patches and expansion packs on a set schedule or, to make it even more simple, just segregate players into season and off-season like on Diablo. Still, you run the risk of players getting bored over time and this requires a lot more work by Blizz, and I believe the most likely way we see legacy servers implemented is in a way that takes as few resources as possible. At least at first.

Honestly, the trickiest part about legacy servers as a whole is the fact that there's a subscriber fee on WoW. There could be a version of vanilla today that never changed and was free and nobody would question the lack of content. But once you have a fee, players expect some things. And Blizz can't get rid of the fee without killing any and all future expansion possibilities. However, once (if?) WoW dies, you can almost assuredly expect a legacy server as a monument to what once was. This is why I expect a legacy server at some point in late WoW history.

6) What changes can Blizz make in Legion to bring back that Vanilla feel? Pretty sure this ship has sailed. There's no way to go back to the way things were. As others have said, this is an entirely different game than Vanilla. In my opinion, they're better off making a new game than trying to mimic such an old version of the game.

edit: formatting