Well I don't know about totally wrong seeing as in the examples you followed up with you only gave Marth the definite advantage vs Sheik's neutral, Falco/Sheik movement, and Falco's recovery. And obviously it was an overstatement on my part.
Also let me point out that although Fox might not kill with three up airs off of a grab it puts Marth in a horrendous situation where Fox can easily get another follow up.
As far as Marth's struggles, he struggles a lot with coming down from the air, he struggles getting out of the corner/off the ledge, he struggles killing various characters if they get to high percent, if you approach with an aerial you are often susceptible to crouch canceling (SDI down/CC shine after first hit of neutral air can be pretty good), and he has a bad shield with OOS options that are fine but not as good as Sheik's 6 frame nair or shine. None of these apply to Fox for example who really only struggles when he gets hit, and every character still struggles in that situation Fox just happens to struggle more.
Fox really shouldn't get those three up airs, though. With correct DI/SDI on uthrow and uair respectively, those combos are more than escapable. Even players like Leffen admit that Marth players just give him free punishes for not practicing good SDI. The upward limit of Fox's punish is certainly smaller than his fellow top tiers. He just excels at punishing mistakes from many angles and can nickel and dime you to consistent KOs at ~100%.
Also, I'm getting the impression you are just listing off stereotypical "Marth weaknesses" every new player parrots. It has always felt irrelevant to say Marth struggles with coming down because almost the entire cast struggles with coming down. Marth at least has counter, disjointed dair, and side-b stalls which is more than I can say for pretty much any non-floaty in the game. He by no means excels by having opponents under him but he fares better than most; even if only marginally.
CCing is probably the most outdated cliche you provided. High player Marths these days space their disjointed, high base knockback attacks. If any character is thwarted by CC at high level its Sheik, imo, but she at least has needles and grab.
OoS options are also uninformed. Marth has a crap shield, but he gets a lot out of lighter shield since his grab and fair OoS has so much range. Up-b is also 6 frames; but its technically 5 since the 6th frame is invincible. This means its "faster" than Sheik's nair OoS. Again, definitely not his strongest suit, but he has way more options than you are giving him credit for and shielding is certainly not the position any character should be using to win the match.
As far as your points Marth does have a very good neutral, but I don't think he clearly outclasses Fox or Falco (or Sheik?). His recovery/weight is not so good that it should be the reason he is the best character. And he doesn't just beat Fox. Actually, Fox is generally considered to have a positive matchup against every single character other than Falco and Marth which average at even, whereas Marth has a much more even match up spread (even extending into characters worse than him such as Yoshi (or link or Ganondorf? Don't know the current thinking on those tbh and they're not really important)).
Idk why you are saying/implying his advantage over Sheik in neutral isn't clear when you just said it was definitive at the start of your post. I also said Falco wins neutral so idk why you brought that up, either lol.
As for Fox, I provided pretty clear examples as to why I believe Marth has the slight edge. Saying you disagree doesn't prove anything. Unless of course you think that "Marth gets CC'd" sufficed as a counter argument; which of course assumes that Marths don't grab, dtilt, or generally space attacks.
I never really agreed with Ethereal that Marth's weight is what made him the best character or w/e. Just pointing out that his attributes clearly put him middle of the road in terms of combo defense among the traditional 8.
Marth/Yoshi is undeveloped. aMSa says its even, so assuming that is true, who gives a ****?. He still beats every mid tier and goes in on all the Mario bros + Samus; certainly harder than Fox does in those specific match ups. And I am convinced that only players who put all their stock in recent results/are desperate for mid tier x to perform well vs top tier x believe Link or Ganondorf stand a chance against Marth at top level. Like, the math of them even being fast enough to compete with a Marth who knows the matchup as well as they do doesn't add up before we even go into actual context. But like you said, its irrelevant.
In terms of relevant matchups, other than puff (whom Marth still beats), I wouldn't say there is a single one that Fox totally outclasses Marth to the point that he is a no-brain superior pick. Even Sheik is becoming questionable since all the Sheik's are learning the tech chase which is appearing
deadlier than the stuff shes been getting off grab vs. Marth for the past 10+ years. Marth vs. Fox itself is a whole 'nother can of worms, but I would say that its pretty clear Marth has the edge in Bo5s since FD, when used correctly, gives him a clear statistical advantage in terms of expected wins and losses.
Edit:
For reference with regard to getting off the ledge at most Marth has 4 frames of intangibility if done perfectly. The only characters with worse frame data than this are Zelda, Mewtwo, and Peach with Ganon and Ness tying (Fox has 15 frames, Falco has 14, and Sheik has 11). A grab will not come out in four frames. His aerials from the ledge are easily punishable with crouch canceling, SDI on the nair, or a little bit of spacing, and even if he gets up he is still in a corner which he is not great at getting out of.
Again, you aren't coming off totally informed. Marth still has hax dashing, fair regrabs, neutral stand (2 frames of vulnerability), a semi- vulnerable ledge drop regrab, etc. He doesn't have incredible ledge options but he can take his time and demand respect from the opponent. Its still a situation Marth can easily turn the tables from and we've seen M2K do this all the time. Yeah, his
invincible ledge options suck but he gets the job done.
