When I got my first character to level sixty five or so, I had the same thoughts running through my head. Going back and doing Onyxia, or Molten Core sure seemed like it would be a lot of fun now that I had a few levels under my belt. The tables had turned! But after ramming my own head into the same wall, I realize it's never going to happen.
Blizzard isn't going to remove the Onyxia and Black Wing Lair attunements from the game. Long after the Black Temple and Mount Hyjal attunement quests are removed, you will still need to clear Upper Black Rock Spire to get into Black Wing Lair and you'll still need to do the twenty-some step quest to get into Onyxia's Lair. Even if you are attuned, chances are good that a lot of people in your guild are not, or are playing a character that never did the quest chains. But, that still leaves Zul'Gurub, AQ40, AQ20, Molten Core, etc. Right?
Let's assume you can't spend eight hours a day playing World of Warcraft. Let's assume you have some goals in the game, like getting some content down, getting your epic mount, or earning some honor. Why would you spend the limited time you have in the game on clearing trash and defeating a boss that won't drop anything that is even worth some gold? Running a regular five man instance in the Outlands will get you more return. You still can't solo those old raid instances, so you will need to get a group of people who all feel it is worth their time and effort to go into the old raid instance and try to down the boss. And, frankly, that's going to be very rare.
And lastly, I know there are people who never set foot in those instances. I am one of those people who never went to Onyxia or AQ. The one time I did go to AQ40 with a full group of level seventy players, it just wasn't the same as raiding Karazhan with my guild or the days when I was helping raid Zul'Gurub for real (at level 60). Going back to do Scholomance was a real eye-opener for me, too. Once you are beyond the level, the raid loses a lot of it's detail and impact. The flaws stand out more starkly and seem to be more obvious.
I still hear it come up every so often. The call to go do some old raid instance. The fact that it rarely goes beyond that and usually never happens is a sign. The real sad part is that it confirms my pessimistic idea that people don't run raids for the content, but for the rewards.