Why not? His 5v5 production rates have historically been pretty solid and he's a great puck carrier and distributor. In the last two seasons, his 1.41 assists/60 at 5v5 would put him 8th among RW's (>1,000 mins), despite playing for some pretty offensively starved Columbus teams. That's ahead of guys like Nylander, Rantanen, Keller, Kane, Svechnikov etc.
His skating, puck carrying, and playmaking abilities make him a very effective compliment when played alongside a strong C+W pair like Aho - Jarvis.
Not everyone on the first line has to be a "first line" player. What I proposed would have 2 first line guys on the top line with a middle 6 guy (Roslovic), and 2 first line guys on the 2nd line with a middle 6 guy (Drury / Kotkaniemi). That gives you more balanced scoring in your top 6.
He has average hockey vision, play bad under pressure, is very dependent on teammates, not consistent, not ideal position play. And again, Jack played in the first line only in the end of the season. If he was as great as you write, we wouldn't trade him (and value was low) or at least the Rangers would resign him.
He plays best with power forwards/grinders who create space for him (btw, Kreider was such a player, Jenner and Kuraly in CBJ. He also played well with Laine. It's true that Patrik led the line, create the play, must defend more because all the players who played with him had problems with defending. Watching Gaudreau on defense was tragedy, he was trying, but I feel like it would have been better if he wasn't defending. Johnson has lack of strength, he was always pushed off the puck, Marchenko had problems in his first season too. I liked Chinachov, but he got hurt quickly.
In short, Jack won't help you in important games. But against teams that don't play tough forechecking, don't have good defense, or aren't in shape, yes.