Originally Posted by
Kralljin
I doubt that even with DF, dungeons would be that popular in TBC, the crux is simply that they're not as rewarding.
Especially as tank you just feel the pinch, it's much easier to just sit in a Kara run as 2nd tank than to slog through 5man dungeons, get some gear and still get pounded by some bosses in Kara (nevermind the threat issues).
Similiar thing would happen in Wotlk as well, why bother gearing through heroics when you can get showered in gear in Naxx?
Reason why heroics remained relevant is because they turned them into catchup content in 3.2 via emblems, at which point people didn't bother about the gear that dropped in heroics but just the emblems.
Disregarding that, Blizzard at least stated that they want to keep heroics relevant throughout Wotlk, even adding new challenges to them as new content get released.