Let's start at the beginning, with Naxxramas. This was a large and excessively challenging raid at the end of Vanilla WoW. To get to it you'd have to clear MC, BWL, and be well into AQ, which was also a very difficult raid. The gear requirements were incredibly high, the consumable requirements were even higher; it was commonly known that the high end guilds were buying gold in order to fuel their raids, since it's likely that otherwise they'd have barely had time after farming. It was in the aftermath of the farming for Naxx that consumable buffs were reduced to only a few at a time. The result of all this, insane requirements of all sorts, plus being very deep into the raiding content (and this was before there were any side paths, so not even heroic gear), was that almost no one saw Naxx. The highest numbers I've seen are 1-2% saw it at 60.
When the devs put that much work into a raid and no one sees it, it makes sense that they'd give it some tweaking and send it up to level 80. After all, for 99% of players it's not even recycled content, it's entirely new. At the least it's efficient, and since it was an excellent raid, it makes sense to show it to more people. Still, I'm not glad that they removed the old raid; I don't believe in removal of content unless it's replaced by something clearly better. In this case there was no clearly better, since the raid was simply gone. It's not the same as when they fixed up Dustwallow Marsh to make it not terrible; that's adding content or replacing bad content with good content.
Onyxia is coming next. I'm no happier. In fact, I'm less happier. I guess that's more unhappy. Why?
It's hard to make the argument that Onyxia was not seen by the playerbase. I cannot even imagine a vanilla raider who did not see Onyxia. As for non-raiders, I can still see them joining a PUG just for that one fight. She was very popular; people were getting her down and getting heads all the time.
Now she's left behind. People don't do the old raids much anymore. Sure, there are nostalgia or achievement runs now and then, but it's not as if anyone seriously runs them to gear up (except a few people who I suspect are complete idiots).
The argument does remain that they are good content and it would be nice to show them to people again. But is this all that good of an argument and even if it is, is this the way to go about fixing this problem? Let's look at leveling instances. SFK is a pretty cool place, but it's not as popular as it once was. Or for a better example, the late vanilla instances: Blackrock Spire, Scholomance, Stratholme, and Dire Maul. People just don't go there anymore. Why not? Because they are for 68+ and when people hit 68 they run to Outland. The gear is better, the xp is faster, and if don't care about instances being bad Outland has plenty to offer.
Should we remake Scholomance for level 80s or 85s? No. That would be stupid. It would be spending extra effort just to butcher a set of zones (the Plaguelands are inseparable from their instances). So then how do you get people to see the good content? Make it more 'profitable.' One of these is easy: give DKs an XP buff when in the Plaguelands. It makes much more sense for them to fight the Scourge than to forget about their revenge for ten levels to go hunt felboars and help goblins cook disgusting bacon. For all players add some badge-style loot for them, such as trading invader's stones for some blues maybe a corrupter's stone gives a really nice weapon. The minor stones would be for XP and the higher stones could be traded down (though at better than 1:1).
Don't remake the zones, just make them make sense to play in. Do the same for the raids. Remove the XP nerf for raids. Dramatically increase the XP from killing raid bosses and trash. Make it so that running a raid will be faster than questing or grinding, enough so that there is incentive to raid, but not so much that people are just logging in for the raids and then quitting afterward. We had enough of that at 60.
The only major changes I'd make would be to add zone-wise fire and shadow resist buffs so that leveling 60s don't have to grind resistance sets. That was not fun and would be a major disincentive. Also it would be much harder now that they've remade a lot of the FR gear to have, you know, useful stats.
We're never going to get back the feeling of the old raids. Even remaking them won't do it. Too much has changed. New class abilities make the old strategies meaningless or even trivial. The player culture has changed. We have new memes and new adventures and it won't go back to what it was. Nothing will make me look at a pair of lava giants at the start of a raid and go "oh my god, I'm here, I am in a raid!" New players don't need those lava giants. They have their own. They have skeletal horses and abominations and dragonkin.
The nostalgia train doesn't go very far. The practicality train is what can complete the journey. Remaking raids isn't the answer, fixing them up for leveling is.