It's video game story writing. The writing and story is shit no matter what. They could have wrote anything to give a reason to update Exodar/Silvermoon.
I don't think they should update it for no reason. There needs to be a gameplay/story reason to justify the time spent remaking the zones.
Sure. It can get the "Darnassus Special". Also known as the "Gilneas Treatment". Aka, "Look What Sylvanas Did". There's an "Undercity Edition" you may be interested in, as well!
If a Silvermoon/BE-themed Warfront happens, the chances of a Silvermoon revamp go way up. The art asset work for the former would make the latter a much easier prospect, though it still would take some work. If we don't get the former, I would not expect the latter to ever happen.
I'm firmly in the corner of bringing Quel'Thalas onto the EK continent, updating it, adding flying, etc. Same with Azuremyst. But without a compelling gameplay (and therefore business) reason, I don't see them doing either any time soon.
If the Silvermoon warfront is going to happen I assume we'll know at Blizzcon and that's the best chance of Silvermoon being updated this expansion. Of course, then Horde won't be able to use HD Silvermoon half the time since it will be a hostile zone in ruins/controlled by Alliance.