Provided that's not sarcasm (tone on the internet is not obvious), I'm very surprised that you think Peach / Marth is so close then.
I really don't find Peach comparable to Sheik at all in the matchup, from talking to Vwins, Cactuar, and a whole bunch of Peaches and Marths. Sheik has very, very good edgeguarding on Marth and the means to do inescapable bull**** edgeguards, stupid throw combos, a faster projectile (faster being the operative word) and the range / speed combination to play a very strong defensive game.
Peach has nothing comparable, except for arguably a superior dash attack, but even that's kind of a meh argument because she doesn't have the speed to support it.
Her edgeguarding isn't real edgeguarding, most of the time, it's usually just trying to make it difficult for him to come back from the ledge to the stage. It's not guaranteed at all, because of Marth's options from the ledge (no perfect ledgedash like Sheik and Fox but he has a sword and other crap and his ledgedash isn't even THAT bad) whereas Peach's options from the ledge are worse than Marth's by a lot and she can be edgeguarded like a normal character too, and if she lives she's probably taken like 40+ damage and that sucks.
@ Kaostar, Peach vs Fox is dumb because she actually can't hit him enough times to win on a bunch of levels but on the other levels where she can, she can make his head explode once she gets inside. Provided, of course, that the Fox is camping and being gay. If Fox approaches, he's fighting an evenish matchup because Peach defensively vs not-Marth is pretty good.
The bulk of your "Marth isn't as good" points, though, can largely be answered by "dash dance better" or "space better". Crouch cancel D-smash blows against Marth. He's never going to be close enough for that to work. Same goes for Nair out of shield. F-throw isn't a good KO move, unless KO at 150% is now considered a good KO move.
@ Raychun1 it's just a dumb matchup because of how much of a grind it is and how Sheik has to play so conservatively to keep a lead. If Puff gets ahead, Puff camping back is really, really hard to deal with because of how crap Sheik is at approaching her. I don't really know numbers, but being able to be Rested out of your only reasonable KO move (grab) because they do what they always do (duck), is pretty balls. Also, Uair combos to Rest on Sheik really easily with Puff.