I would love to hear reasoning for this. As I see it, Fox has more room to progress compared to Sheik and at the moment Fox definately> Sheik.
It's mostly got to do with Sheik's defensive game.
Sheik crouches. Because of the crouch, Fox can't do Nair approach because if he does he gets CC grabbed and reaction-based tech chased to death, or gimped. His only way to do Nair is low Nair, which is hardly a good approach, because of the lack of active hitbox. Bair is eliminated similarly.
If he wants to approach, he has to grab or use the Shine (risky -- she has better range than both of these) or Dair her. Now envision a Sheik that, in addition to crouching, SDIs the Dair away to avoid the Shine and other follows.
Now say that Sheik can SDI the Uair in U-throw Uair without needing to DI the throw, which means she can't be comboed very heavily by Fox's U-throw game.
The logical solution to such heavy defense is to just camp, since lasers are fantabulous, but Sheik isn't slow enough for that to win a matchup by itself, and she has a lot of potent range at her disposal too. She can also ledgecamp and use her own projectile if she's winning. Most importantly, her movement and stage presence enables her to catch Fox, whereas other characters with similar defensive potential (Peach, notably) cannot.
Now, I'm sure you think I'm completely insane for thinking that there could be players that can do this. But, really, when you look at some of the high level players existing, I don't think this is exactly absurd. People SDI the Uair in Fox U-throw Uair routinely, especially players with good reflexes like Mango. Axe's Pikachu also gets out of it with high frequency. For Dairs, S2J reputedly is good at SDIing the Dairs in Drillshines to avoid follow with Falcon. If we can combine these principles and apply them to a character with a really powerful crouch cancel game and ground game (Sheik tilts, D-smash, grab range) to nullify the Nair portion, we effectively shut off a lot of Fox's damage output.
In contrast, Sheik's combos on the fast fallers are based off reaction time -- nothing more. If Sheik develops accurate responses to the tech, non-tech options and routinely answers them accurately, she basically gets to play Solitaire with Fox's body because he can't get out on his own. He needs her to make a mistake. We already see large combos based off reaction time with Sheik now.
I'm not sure whether this actually will happen, or even if it could happen, because admittedly doing all of this together is a tall order. But it is a thought I had.