The move only lasts a long time if an opponent is being hit by the attack. Even assuming a best case scenario, where an opponent DI's out perfectly on the second hit, and the matches momentum is 'reset' as you say, the opponent is still in very close proximity to pikachu. What your missing is that this situation is incredibly favorable to a pikachu player, especially moreso then if they flew away. They are able to keep the pressure on, and momentum would only be reset if the player chose to do nothing. In Brawl (even moreso then melee) Pikachu's close range attacks have awesome speed, priority, etc. Having the opponent next to me even if the conditions were completely neutral is a gift. The biggest problem might potentially be characters with some range attacks like Marth, Ike, or Pit, who might be able to keep pikachu away, although I don't imagine it would be since even theyre still rather close to plan much and theyre so close that a QAC -> dair, uair, nair would hit them.
Also, that's assuming a best case scenario. If the opponent is tapping straight up, then they'll leave towards the end of the attack. If they slide the analog stick along the top of the pad, they could push out much earlier, but the hitstun from electric attacks leaves you with at least a 50% chance (if not more) that you'll fall back into the attack and be hit by the last part of the attack. And since they're at the maximum upper part of the range (thanks to their upwards DI), it has a greater chance of sending them straight upwards or in that general direction, and that is probably the only useful time the dsmash sends an opponent far away.
On top of that, after a certain percent (depends on the character) the opponent will have to be good a teching because itll send them crashing into the ground very quickly and at whatever random time the DI out. Even if they do tech, it's just extra time pika has to set up for a usmash, forward smash, nair, or some other move.
I think the only people this will affect are people who liked to camp pikachu or don't know how to use him aggressively. For people who know how to use him aggresively it shouldnt be too much of a problem.
P.S. I realized you probably don't know, the dsmashes main use is as a defensive tactic, almost like a shield. Forgot to expand on that oh well.