How are they better than him? Too early to say but I doubt any of your players have performed as well in their first 20 games as he did.
Kid will be a star and your not getting near him
Travis Boak has averaged 22 touches, 4 marks and 4 tackles a game over his career, while copping a hard tag a majority of the time, winning Port Adelaide's best & fairest last year and also has a top 10 Brownlow finish under his belt.
Robbie Gray is a small forward who can also play as an excellent midfielder, being our leading goalkicker last year. A small forward who can kick bags of goals (6, 5 and 4 numerous times) is rare as -insert something rare here-. He's averaged 15 touches, 3.8 marks, 1.5 goals and 3 tackles per game. In his first 21 games, he also kicked 32 goals, just like Menzel.
Daniel Menzel is 5cm taller than both Boak and Gray, so can play as a marking forward, yet only averages 4.4 marks per game, barely more than either of Robbie or Trav. For primarily a forward (as he only averages 11.9 touches per game), his goalkicking average is the same as Robbie Gray (1.5 goals per game), even though Robbie spends far more time in the midfield. His 1.7 tackles per game also suggest that he isn't the king of forward pressure either.
So you might have fall in love with Menzel's potential, as a lot of people have (I think he has the talent to be a star if his knees don't screw up), but Robbie Gray and Travis Boak are both far more proven and far better players than Menzel at this stage.
Edit: Basically, you just proposed we give up two of our Top 5 players for a player that isn't in your Top 10 currently.