Do you want them?
Absolutely. Keeping them separate from current expansion is important though as to still progress the game and story. Adding Legacy servers could be a vital way of getting more sub revenue as well as giving us a chance to experience it all over again (or letting newcomers experience it for the first time).
How much would you pay for them?
I saw another suggestion on here that would be perfect. If it can be included in the $15 monthly sub then it should. If the mega corporation requires an extra amount to fund the hardware and development teams assigned to these servers; an extra $5 would be a great way for us to be immersed once again.
Will blizz ever put them out there?
Absolutely. It is only a matter of time. But suffering the loss of Nostalrius and the backlash it has created so far, we are all hoping to see them relatively soon.
Why do people want Vanilla back?
Because it is the ultimate MMO from Blizzard where not everything made sense and it was awesome. Agility and strength on a cloth item? Vanilla WoW was one, if not the biggest success and reason that Blizzard became the giant company they are today. They owe that success to Vanilla, BC, and WOTLK. It was an epic journey in its time where you might follow a quest from Menethil Harbor on a boat to Theramore, and then run to Desolace to give someone a piece of paper. And that's the rest of the quest! But you explored, you couldn't fly, and you may have been ganked by mobs or mean hordies. Even though the quest didn't sprout off into other quest lines, you at least explored and had a fun/challenging time doing it. Giving us the option to return to the glory days is another vehicle for Blizzard to drive up sub numbers and really create a positive environment that the community can be proud of.
Why would legacy servers NOT work?
The bottom line is that they CAN work. There is nothing that can keep them working unless Blizzard just refuses to make them work. Anything can be fixed/tuned up to adjust it to make it into something the community can enjoy and be truly happy.
What changes can Blizz make in Legion to bring back that Vanilla feel?
Nothing. It's a very simple answer. You may take out LFR, LFG, and create more social aspects. But they have made so many quality of life changes, changes to the world, changes to questing and dungeons, it will never feel like Vanilla again. They should embrace that and continue to move forward. But they should not turn their back to another avenue that they could be successful with.
All in all, Nostalrius was the happy place we went to in order to experience the magic all over again. It reminded us why we still playing this game, even in its worst state. We all hold on to this idea that one day it will return to the way it is but it won't. Legacy servers are the best way for us to get back to the way things were. Like others in this thread I truly believe that if we keep shouting they will eventually listen to us.