Coming in late here but I want to chime in. As a prospect - Hughes isn't McDavid-level, and isn't Crosby-level either. But your question is more "can he get there?"
It's easy to compare prospects to current stars but hard to project just how good players will be. So most any assumptions about what kind of player Hughes will be are guesses - no matter how surefire of a prospect he is.
That said he's an elite prospect. When people say that he's "not McDavid or Crosby," that's more of a nod to how good those two are rather than a knock on Hughes. Watch McDavid play. He's just so much better than everyone else - he clearly does things no one else can do. The combination of his shooting, passing, hockey IQ, speed, edges, vision, and size are unmatched.
But I'm going to use your post as reason to simply explain why I think Hughes is an future NHL superstar despite not being quite Crosby level. There are a few things that set Hughes apart and make him an elite prospect.
First is the one everyone knows about - his edges. His skating is absolutely dynamic, it sets him apart from almost every other prospect in the past few years - aside from maybe well...Quinn Hughes. His first step is top notch but his overall top speed, while very very good, isn't the best of the best. It's Hughes' speed east to west, the speed at which he turns on his edges that creates space for him on the ice.
Watch starting about 0:29 in the following video. Watch how Jack enters the zone with speed and moves east and west across the ice and changes directions while beating everyone else:
The next thing about Jack is his brain. This is something that doesn't get talked about enough. Watch the video below starting at about 1:00.
Jack might seem a bit blank in some interviews but don't let that fool you. This is a really smart kid. It's not just his physical talents - he really understands the game and is able to make smart analysis like the above while at top speed. I also think it was Pronman who pointed out how in conversation with him, Jack was able to recall details of plays from months before.
Watch below at 6:27. He knows where players are going to be before they even know.
At 2:29 of the above video you can also see Hughes enter the zone with speed, spin and hit Kane with precision.
Then watch below at 3:30-4:00. We could do vision/passing stuff all day.
Don't get me wrong the kid can shoot too, watch below at 2:50
Hughes is also really brave. He''s not strong but he's fearless. He'll go anywhere with the puck and fight for the puck anywhere. Watch the skinny freshly minted 18 year-old dive to steal the puck from Ovechkin at 7:01 below.
Hughes is really unflappable. This is important to mention because it not only compliments his skill on the ice, but NHL teams, like the Devils, see leadership qualities in him. This might seem like a joke but watch the video at about 6:30 as Jack introduces himself to the viewers.
This is just one of many moments where you can see just how comfortable jack is with himself and how confident he is at such a young age, especially compared to Turcotte before him. He can come off cocky at times, but if he didn't have a little bit of healthy ego, he wouldn't be the player he is today. He's not faking his confidence, he believes he can be the best and that's important as far as reaching his potential goes. And that's not to say he doesn't have any humility, I think he has displayed it in most every interview he's done. Well raised kid.
And then what makes Hughes a top flight prospect - is that he takes all these skills, the ones he does elite and the ones he does just very good, and does them all together at speed. There are many players who have better handles. There are many players who are faster on a straight line. Many of those guys aren't superstars. But Jack has a real shot at being a superstar because everything that he does very well, he can do while going at full speed.
Watch at 6:48 below as he reads the play and intercepts the pass at speed, slides on his edges to create space between himself and the D-man and then feeds a pass Hanifin who scores.
So, can Hughes reach Crosby-level? Well, I don't know if he ends up quite impacting the overall game as much as Sid. But it's hard for me to watch Hughes' IIHF World Championship highlights and not think that he has the potential to hit 100 points multiple times in his career with the right supporting cast around him. A lot of people raved about Kakko at the WC. I thought Hughes had a great tournament - he might have been even better than Kakko but it's hard to compare two very different players. People had such high expectations of Hughes that they seem to think he played poorly at the WC. But watch the highlight video above. That video accounts for a lot of his ice time. He was dangerous almost every shift - sometimes the most dangerous player on the ice.
Ultimately, there are three main knocks on Hughes:
1. Size and strength - this is a legit issue. I think he is going to have a lot of trouble winning board battles next year. Ironically, Kakko is a juggernaut on the boards. Hughes has to get stronger and fans need to allow him time to fill out as well and not freak out when there are bumps in the road next year.
2. I've heard a bunch of scouts/writers say that when his team is struggling, Hughes tries to do too much. Obviously this isn't a big deal and hopefully Hughes can learn to be disciplined here.
3. He had an amazing supporting cast with the USNTDP. - This is obviously true. Turcotte, Zegras, Caufield, etc...his team was stacked. But who cares? In the NHL you also need a strong supporting cast to have success. It's not like Hughes doesn't have undeniable clearly observable elite skills with or without great players around him. I'm not worried here.
Overall, I think the player we are getting in Hughes and his potential is exactly what you are implying in your question - a notch below players like Crosby and McDavid, maybe even Mackinnon, on par with some elite players like Matthews, and well above a Hischier. These things are impossible to predict with certainty but I personally wouldn't be surprised if he's in the top 10 leading scorers in the league for the majority of his career.