I'm not sure why you think the only people wanting to do speed runs are legacy players. Legacy players are just as new to the dungeons as everyone else.

The people you're seeing aren't all legacy players; they're "get to the last 1% ASAP" players that jump from MMO to MMO. They did the same thing in GW2, they're doing it here, and they'll do it in EQN. Give it a month and they'll be tired of making threads complaining about NOT ENOUGH CONTENT!!1 in the game's first week of release and leave.

Then you'll be stuck with the actual legacy players, who are an awesome group.