Also, how does Mario struggle to kill? All his smash attacks are fairly dependable, bthrow can kill at 160-170 if kept fresh, uair is fairly effective, and bair can kill in the 140-150 range. These are just his move kill options; keep in mind that he also has some of the best edgeguarding tools in the game with not only the cape and FLUDD but also one of the best ledge trump bairs.
140% and above are not good kill percents.
His smashes are safer than a lot others but they are still punishable on whiff. D-smash is weak, F-smash is unsafe unless spaced (And if you hit a shield at all), U-smash is reliable, but it comes out from behind Mario and has a blindspot on its front.
Mario's ledge trump B-air is... bad. If it was one of the best, Luigi's would be overpowered. Mario also tend to struggle edgeguarding characters who recover below the ledge, which a lot of character prefer.
I think the reason for nerfing sheik and ZSS to make more low tiers valid/viable is dumb and decreases the meta potential. There is no reason to nerf characters that are highly technical and interesting to learn and play (but not always play against). As stated before, sheik really only needs some nerfs on her recovery to make it less safe, but no other nerfs should be considered for her IMO since she does not have nearly the same super-abusable tools that other high tier characters had in melee or brawl (i.e. melee and brawl marth's range basically invalidates and shuts down any and every mario and luigi's approach games completely X__X).
Sheik has needles that invalidate some characters' approaches, Sheik has a frame 5 F-air that stretches 'till the horizon... Toning down those, like I said, will not move her to the trash can. Nerfing those are completely fine because they aren't adding depths to the character, they're just plain dumb moves that shouldn't be there in the first place.
I'm sure a character with excellent mobility, damage racking ability, and safety is not balanced when they are also equipped with the best camping/zoning projectile in the game and an extremely fast aerial that outranges a sword on a character with outstanding SHFF game and a very generous AC window.
By the way, nerfing one or two characters are way easier than buffing 30+ characters. Nerfing Sheik (And perhaps ZSS)'s main goal is, IMO, NOT to make low tiers viable, it's in order to erase those 'REALLY high in X's favor' or '80-20, maybe 70-30' matchups. 6:4 means the disadvantaged character needs to work harder, but winning is still highly possible. Winning 7:3 MUs are kinda rare in top level unless there's a huge skill gap or matchup inexperiences (Take Luigi vs Sheiks for example).
Besides, buffing every low tier to high tier level is a tall order because each character need thoughts to actually rise them up on the tier list. You can't just add in 1% on some random Zelda's moves and expect her to take a regional tommorow. Each character is different, and buffing one tend to differ from buffing another. Even the seemingly simple buff of Hoo Hah needs some thought. Will the kill setups be too overpowering for low to mid players to fight against? How often this characters will get grabs or how good this character's grab is? Those are just a few examples.