You're not going to replace Kessel's production most likely if you trade him, I don't think anyone really expects that. You may be able to replace Kessel by committee, with Schultz taking over his spot on the PP and getting a better ES player than Kessel in that trade, but no one will individually be able to replace his production. However, you don't need to replace his production to end up a better team by trading him. There are 3 massive problems I see with Kessel on the Penguins:
1. He doesn't fit well with Crosby or Malkin, and he doesn't want to play on the 3rd line.
2. His play away from the puck has always been really bad, to the point where he's actively hurting you if he's not producing.
3. Kessel has already started to show some decline this year, to the point where I don't want to be the team holding the Kessel bomb when it blows up. It's not an if, it's a when. Kessel is going to fall off a cliff at some point, I just don't know when that is.
In 2019, Kessel only has 7 goals and 27 points in 36 games. Of those, only 3 goals and 15 points are at ES. In this sample size, he's on pace for 7 ES goals and 34 ES points per 82 games, while being an even bigger liability defensively than he normally is. His powerplay production isn't even close to worth it if this is his new normal going forward. Kessel has good stats on the year because he had a really strong start to the year, but he has been horrendous the last 3 months. I would pick the safety of trading him, when you'd probably get a good return for him, over gambling on his last 3 months being his new normal going forward. If he fit better with Crosby or Malkin, that might be different. If he wouldn't be so bad away from the puck, that might be different. But it's a combination of all of those 3 that makes him a must-move unless he has an incredible run to end the year.
Basically, I view Kessel to have multiple red flags and keeping him isn't worth the risk for a team that can survive without him. The risk of him falling off and becoming an albatross, which is completely possible if his new normal is what he has been for the last 3 months, is worse than them prematurely trading him IMO. The Penguins don't lack the top end talent for the gamble of keeping Kessel to be worth it.