They have Connor, Ehlers, Wheeler, Copp and Stastny as wingers capable of playing top 6. Harkins is currently the #10 in aTOI for Jet forwards with 10:54 and has played almost exclusively bottom 6 minutes (I see daily faceoff has him as 2RW, but he doesn't play there unless the coach is shuffling things around).
While Hertl would be arguably the second best winger on the team, he is not enough of an improvement to justify the high price that other teams would likely pay for him to play 1/2C. Not because he isn't a great player, but because the Jets need for him is a lot less than other teams. If the Sharks were willing to move him for cheap however...
sarcasm:I realize that isn't happening)