I partially agree with this assessment. Low tiers are low tiers and will struggle to overcome matchups against the two best characters in the game and the very bizarre Rosalina.
However, I also believe that Sheik is a completely different beast from the other two. While I agree that all three are holding back low/mid tier viability to an extent, Sheik is holding back a LOT of viability on all tier levels, primarily due to how she's designed as a character.
My main gripe with Sheik that is not present with ZSS and Rosalina is that she doesn't have any inherent character weaknesses or losing matchups. ZSS, as I mentioned earlier, has a lousy grab, a really bad UpB recovery if you can force her to use it, and has problems against low crouchers. Rosalina, on the other hand, has a very easy to intercept recovery, and becomes no more than a mid tier character if you're efficient at picking off Luma. While yes, I will agree it's rather difficult to fully exploit their weaknesses, the fact remains that they ARE there for some level of counterplay to exist against them. And, within the realms of high/top tier land, those two do have a reasonable amount of matchups that they struggle with, and even lose.
Sheik, on the other hand, isn't held by these standards. It's already accepted that she doesn't lose any matchups, but rather goes even with a few top and high tiers. And the character weakness she was initially designed with - being unable to kill - really isn't there when you factor in her Dthrow > Uair mixup, offstage Bouncing Fish combos, and Vanish and Usmash traps. I would definitely say that Sheik's ability to kill is average, at worst. And when you tack that on to everything she already has going for her: one of the best neutral games, one of the best camping games, one of the best recoveries, and one of the best run away games, it can really be a mite overwhelming...
At the end of the day, the reason I want to see a character like ZSS on top is because she has her own inherent flaws. Even if it's difficult, it IS possible to play around her advantages and use what works against her. With Sheik on top, the climax of the metagame is literally "Pick Sheik, because nothing outright counters you" and that's the end of it. I am deeply worried about this conclusion occurring after the patches have ended, because I know this game has the potential to unlock a much larger world of depth.
It's literally just one step away.
(Also just putting the disclaimer out now, I DO NOT want to see Sheik nerfed because I struggle against her to some extent, because then I would want to see Kirby nerfed too and that's just silly. My reasoning is that I legitimately am concerned about her turning into a Brawl Meta Knight - although not as bad - where she just kind of just... turns into the metagame... if the patches have concluded and nothing has changed about her.)