Yeah, I misread your post. Sorry about that. The thing with Diddy versus Fox's and especially Falco's recovery is that Diddy's Monkey Flip is difficult to challenge since it's basically a flying sex kick or a flying grab. Can it be punished? Yes, but no competent Diddy will land onto the stage unless they have a valid reason to do so. Fox on the other hand can be hit out of Fox Illusion and Fox Illusion can't kill or gimp like Monkey Flip. Fox Illusion is also a bit easier to challenge since it has as set, linear distance and it can be clanked while Monkey Flip could end up with someone eating a kick if they mess up or the Diddy figures this out and uses the grab instead. Also, Fox is a fast faller with an average jump and average air speed. By the time he glides back from the far, upper corners of the stage, he's probably going to be below the ledge if he doesn't use Fire Fox or Fox Illusion to shoot towards the stage. Falco's in a worse situation because of Falco Phantasm having no hitbox on him, front hitbox, or a hitbox on the later end of Falco Phantasm. It's really easy to challenge his Falco Phantasm and even easier to challenge his Fire Bird since it travels much shorter than Fire Fox. At the same time, Diddy suffers if he's forced to use his Rocketbarrel Boost to recovery vertically. If he's doing it diagonally or horizontally to place a hitbox you don't really want to challenge, then Diddy's Rocketbarrel Boost is a stronger, but a more manageable recovery since he has to charge it meaning you can tell where he's heading.
So, Fox and Falco not entering helpless mode with their Side Specials is eh. I think it's there because of the SD issues where you can Side Special and shoot off the stage which I think Ganondorf should have had the same treatment or make it so his grounded Flame Choke doesn't slide off the ledge like Captain Falcon. Does it change much? Not really since Fox and Falco do the same thing for recovering: fall back near the ledge and use either Up or Side Special to recover. I don't see a lot of people Side Special then Up Special to recover since Up Special for recovering is more punishable, especially Falco. On the ground, it doesn't do much since they still stop at the ledge. And if they get hit, they're going to use either of them again just like in past games. The only Star Fox character I can see "needing" this is Wolf because his Wolf Flash traveled in an angle making SDs much, much more easier. You just have to Wolf Flash once and you'll die as Wolf if you're too close to the ledge or if you end up helpless and getting punished. Fox and Falco at least need to jump before shooting off the stage. In terms of gameplay, well, it lets Falco use a situational soft spike and Fox can Wolf Flash spike people that needs the right angle and right timing and they can sort of combo with them... that's pretty much it. Nothing insane. Diddy, however, can do more with Monkey Flip and I wouldn't be surprised if Monkey Flip footstools - the thing where you jump during Monkey Flip's grab - become a valid edgeguard for Diddy. Seriously, Diddy would just Monkey Flip, meet you halfway, and unless you're Villager, Jigglypuff, Kirby, the Pits, or Meta Knight, you're probably not making it back on stage. Or he'll kill you with a Monkey Flip kick off stage. If Diddy entered helpless mode after (a whiffed) Monkey Flip (kick), that would deter people from using it like that, but if someone can get the right timing, it's not going to mean much.
Sheik on the other hand is safe with everything. Didn't Vanish have I-frames in past games too? Bouncing Fish just makes it even more ridiculous since it's difficult to challenge. Incredible air speed, good jump, quick aerials to cover, and safe recoveries. Sheik might have been intended to be a "safe" character, but it's kind of ridiculous at times. Bouncing Fish is essentially a low-angled, can't spike Flip Jump and ZSS doesn't end up in helpless after Flip Jump. The difference? ZSS's recovery is going to be focused on Flip Jump and tethering since Boost Kick doesn't travel that well vertically. The high arc means ZSS would end up over the opponent while Sheik's going to meet head on and she's likely going to hit the opponent. Vanish allows her to go in 8 directions, can kill with the vanishing part, and the reappearing part has a windbox (that can kill Zelda if she uses Din's Fire near the ledge). Much safer than ZSS's recovery. No I-frames or reduced I-frames and/or increased end lag if she ends up on stage would make it more punishable, but the design or how Vanish works makes it a strong recovery. Here's the thing: would a weakened recovery do anything to Sheik? Smash 4 Sheik is essentially a not broken Melee Fox: a character so good on paper and with so much potential in the right hands that massive changes would have to be done to destroy her and even then, it might make her stronger. Adding knockback on her Fair to make it less comboable could allow her to gimp people much easily with the knockback, making Bair weaker took off a kill move, but that doesn't stop her from being able to combo with it, rack up damage, and kill with Bair's new KO percent or with another reliable kill move. Sheik would have to be butchered as a character; massive landing lag and ending lag, reduced range on Needles, massively reduced kill potential, and massively reduced damage output. That would destroy her character design and would make her borderline unusable.
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Realistically I think the nerf you described would drop Sheik to the bottom of top tier or MAYBE the top of high tier, she would shut down fewer characters completely but you still have the excellent frame data making her hard to touch as Dabuz alluded to.
