I feel something like a tight ilevel sync would be more effective, since whatever numerical values the dungeons would be given to challenge a level 60 character will slowly but surely be outgeared once again. Syncing players to the minimum (or perhaps even lower than minimum to compensate for having skills up to level 60?) ilevel for an instance would help keep its charm and challenge intact.

Properly incentivizing such dungeons would help, something like a long term, cumulative glamour or mount grind, maybe toss in a title or two or a minion, something that will make players want to have another go, rather than trying it for the novelty of something different, then shelving it along with all the other old content. Regardless of how exactly it's implemented, I would enjoy having a reason to re-visit the level 50 dungeons, and having things fight back instead of just melting beneath our feet would be a welcome change.