So what to do? We're sure Blizzard will come up with a way to make these instances relevant again-- in fact, I'd be surprised if, despite what they said earlier, they weren't already working on it now. But just to help them, here's a few suggestions of how to fix the old instances.
-First, and most obvious, is enabling Heroic mode. We already have dungeons in Outland that will be able to be tuned to Heroic mode, which means bosses and mobs are harder, but give much better drops. In the same way, Blizzard is likely going to let us optionally turn up the difficulty in UBRS or Stratholme, offering better loot for a bigger challenge.
-Send us in for quests. This isn't quite the same, but it's another likely option. During the AQ gates quest chain, one section took level 60 players back into Sunken Temple to watch a special cutscene, and the same thing happened in Scarlet Monastery with Naxx. A few Outland quests are already sending players back into Azeroth to finish them, and Blizzard could do the same for the old instances, offering special summons or cutscenes (even involving challenges, like the 45-minute Baron run, or the DM Tribute run) inside the old stomping grounds.
-Tune them up. Instead of leaving the old mode in and offering Heroic mode seperately, Blizzard may just tune the old instances to the new content. What if MC was tuned down to a 25-man instance that offered epics at a 62 level? Or ZG became a 10-man instance that offered epic rewards for level 65s? On the way up to 60, players would be much likely to make a go of either one. Unfortunately, this plan means newer players will never likely see the "old" content in the form it's now-- a large part of the player base has never seen a 40-man Naxx, and if it's tuned to 25 players, all those new players will never experience taking down Kel'Thuzad with a full raid.
No matter which way Blizzard goes about it, it's very unlikely that these instances will serve a real purpose at the current time (of course, people could always just run them for fun-- maybe I'm giving that idea too short shrift). To tell the truth, the gear in the old content is not that bad-- I really like my Earthfury set on my Shammy, and while I've seen a lot of good loot in Outlands, I haven't found an armor piece that makes me break it up yet. But to get that set, I had to run MC again and again and again, and at this point, it doesn't seem that there will be enough people hanging around 60 for long enough to run 40 players at a time week after week through MC.