G&W doesn't have parachute Up-B because all slow floaty and high mobility recoveries have been removed. Think about ROB, Pit, Wario, or Toon Link. All of those characters have had major recovery changes similar to G&W. This is a design choice that forces players to think harder about the consequences of their recovery decisions. Whereas in Brawl those characters could often use a recovery move early and still wait it out if they didn't like what they see.
The Brawl Nair vs. Melee Nair argument is a little more complicated. They are both unique moves in their own way. They both cover a lot of space in a way that no other move in the game does. You could argue that the original intent for Melee nair was probably "because melee," and that's probably true. But after several versions it's stuck around for a reason. You have to look at the two moves from a character design standpoint.
Brawl Nair covers a huge spot and forces the opponent to respect it and move away. It is epitome of a zoning tool, which was cool for G&W in Brawl. Zoning was good in Brawl. The physics (especially on G&W) meant that spacing moves and retreating out of the opponent's zone put real pressure on. In PM that wouldn't work at all. Even with L-canceling, characters are too fast for zoning moves to work. Just try and use G&W's back air (a similar move) like that in PM and see how far it gets you. No, in PM a brawl-style nair would be used for juggling. It would be amazing for juggling, probably too good in fact. No other character has a juggling move that hangs around for longer than an air dodge. There would be no counter play, since you can't out range it and you can't avoid it characters would just be stuck in the nair vortex for fifty, sixty, seventy percent.
Meanwhile, Melee nair is straight forward. BOOM, hold that. It's a combo finisher that does a lot of damage and put's the opponent in a bad position. It doesn't bring up any significant balance problems and meshes well with the rest of G&W's play style.
The PMDT doesn't make changes on a whim. A lot of thought gets put into every piece of character design. G&W is no different, and I think, especially in 3.5, he's a solid character that is unique, fun, and has tournament viability.