It's a silly move that feels very janky, and with the history of other characters, everyone else tends to go a different direction than where MK's Dair is design wise. If you use it offstage you're basically dead, which would be cool if it guaranteed that the person dies BUT IT DOESN'T! Onstage, it's a cheesy approaching/getting down tool that usually forces the other person to kind of sit there and not doing anything because "Captain *******" might fly at you OR straight down OR away in the other direction OR he may just keep jumping there or naturally fall.
One move is creating an ambiguous landing game for no reason, instead of being IDK an actual aerial? Almost every character from Brawl with a Dair that plummets you, had it changed. Sonic kept his but they added useful properties to it and he can land it out of combos offstage for fun times. Wario got a new Dair, but his works much smoother and is much easier to balance. Not to mention you can actually try it offstage and not die for landing it. TL and Sheik got reverted to Melee, which is good, but you could apply your argument to those cases as well. Why not try to tweak them in their Brawl state? The answer was more than just "bcz Melee". Those kind of Dairs aren't popular and are very hard to make satisfying for both ends. How would you balance Kirby Stone or Bowser Bomb, if you turned them into Multi hit moves that you can also angle (and in Stone's case, couldn't cancel out of)? Same thing with Ganon and Falcon. These characters have alternative properties that make sense (even if Falcon Kick from the air is pretty crappy overall), and if we were to start tweaking them it would probably be easier to balance than judging what in the hell you do with MK's Dair as is. On top of it all, MK is using his Dair slot for this move. He could be having something more like a regular aerial, instead filled with this. If you replaced all of those character's Dairs with their respective Down B from the air, you'd have an outrage and it would feel silly wouldn't it? Imagine Captain Falcon doing a multi hit foot dive instead of his stomp, it's an awful thought.
For extending combos, you could come up with something else. You could use a Meteor, a Mario-like Dair, maybe even something like Snake?
I mean, if you really want you can try to balance and fiddle with his current Dair, but it would be so much easier (and probably really satisfying) to take it a different direction and find something cool. You could try the Brawl Dair with tweaks, a multi hit Sword Slash Dair kind of like what Snake does but possibly not sending them down, a whirling spin like Mario or Squirtle, something like ZSS's Dair kick, etc. There has got to be a more satisfying alternative than Hasbro's "My Little Kamikaze Plane".