You're right, you can't trade JT or Matthews, that's a terrific 1-2 punch down the middle.
You have little cap space, but unless they're trading Marner for a guy like Jones (which CLB won't do) then they aren't improving, they're getting worse and hoping to improve later.
Nylander is on a cheaper (not better) contract because he's not as good as Marner. Marner leads all Leafs forwards in SH TOI/GP and is 3rd among forwards in PP TOI/GP. He's an all-around player that has elite offensive skill.
Also, I don't think I'll ask Calgary fans anything because he isn't Mitch Marner so they can't really know anything about it. In 6 season he has a single 90pt year and an 80pt year. Marner has a 90pt year and was on pace for another 90pt season this year but due to injury and covid19 he didn't get to finish it. Half his seasons are 90pt years or ones where he was on pace for it. He's more expensive because he's a more valuable player. No disrespect to Gaudreau because he's fantastic but there's a reason he has that contract.
Lastly, with all of that said I know Marner is overpaid but being overpaid doesn't mean Nylander and Gaudreau are better players.

No one is saying those guys are better.

Also, I think Marner is elite on special teams, but I am not always impressed with his 5vs5 play to be honest.

And he did not show much in the playoffs so far. And if you want to blame someone for the goal yesterday, he would be among the players to blame as he did not bring the puck deep.

The point of it all is that you keep your best and only trade them if you're getting a better or equal player(s) back. There are very few trades for Marner that make sense for the Leafs. I'd keep him right up until the final year of his deal and I'd only trade him then if he starts pulling off the same BS negotiation tactics that he did previously.

No one is saying those guys are better.

Also, I think Marner is elite on special teams, but I am not always impressed with his 5vs5 play to be honest.

And he did not show much in the playoffs so far. And if you want to blame someone for the goal yesterday, he would be among the players to blame as he did not bring the puck deep.

Keeping Marner will lose some players of Hyman, Rielly, Andersen, not being able to pay others like Sandin etc.

The best players do not always form the best teams

The goal was scored on a bad line change. Rielly and Ceci were late off the bench.

You forget that Dubas is a genius.

And who did not manage to bring the puck deep in the zone?

Can you explain why?

Where were his genius moves so far? Only trading down for Sandin was

Keeping Marner will lose some players of Hyman, Rielly, Andersen, not being able to pay others like Sandin etc.

The best players do not always form the best teams

Ask the Dubas fanclub, they're many here. I agree that he is over rated.

Not saying he is bad yet, he is just not in the league long enough. He has not built a contending team yet. He may be seen as a great GM one day, but I still think you have to prove yourself first and be called great after