Considering the Melee top 8 has been enjoyed for 13 years and the tournament numbers are higher than ever I don't think it is even contraversial to say most people would enjoy the top 8 staying the same more than if they were changed. Most people obviously see them as fun to play as is. I think also that this fact alone shows how well designed the characters are for this to be the case.
I really don't understand how anyone can be so confident that they KNOW what other people want. Are you a marketer?
Why does this make it good design, because people like playing them? People like playing them because they have the most power and meaningful choices. Fox has so many options and all of them are powerful in their own way. Using the correct one requires foresight and finesse, which makes it feel rewarding. However the fact remains that he possesses a lot of raw power and flexibility, arguably to the point of excess.
The roster needs to be designed so that every character has a niche. With such a large cast, that is difficult. What is Ivy's niche? Defensive spacing with punishing combos? What about Zelda's? Defensive spacing...without punishing combos? Are there any other niches to fill with regards to Defensive spacing? Probably, but I don't think we are currently filling them. Are these characters currently filling them? Perhaps, due to Ivy's reach she can try to space offensively, and due to Zelda's teledash she can try to be aggressive. But they are not as good at these things.
The Melee top tiers have a lot of flexible power, and their "niches" are quite large. Fox can do many different things: approach, camp, gimp, recover, combo, space, and kill. He is the all-around character, and the only characters who can beat such a thing need to have a better strength in at least one of these categories, just to even be worth playing. Falco has better approach due to lasers making it safe, as well as stronger combos (pillaring), but without as good recovery or kill power. Marth has better spacing, comboing, and killing, but no camp and little recovery. Sheik has great gimp and good combos, but with bad kill power and recovery. Peach has good spacing, gimping, and recovery, but bad approach. Jigglypuff has excellent spacing and recovery with good gimp, but bad approach and camp.
What I am trying to say is that the Melee tops filled huge niches, as if the game only had a roster of 6+some (ICs for pure combos, Samus for camp/kill, etc). But in a game with 30+ characters, flexible characters can't have such high power levels. In order to make a character as strong as Melee tops, their strengths (aka jank) have to be so overbearing that they have a niche: abusing that one strength. To make Sonic as powerful as Fox, he'd better abuse his speed to approach and combo with little hits to all hell, because that's all he can do. To make Ivy as powerful as Fox, he'd better outspace and gimp everyone with those huge vine hitboxes, because that's all he can do.
Fox can afford to be flexible if the whole cast gets their power level torn down, but honestly Mario should be the all-around guy, so it would be better if Fox were redesigned to fit some other niche. Maybe the all-around guy who's fast but has less kill power or something, still close to old Fox. I don't know. But he is not well designed (not that I think Mario is either)