I think there are additional explanations besides Sullivan seemingly wanting wingers to fail.
The Penguins play an up-tempo system. It requires all five guys on the ice to not only play fast, but to think fast. Quick reads, quick decisions, quick passes. It's why some defensemen have failed here, too. They couldn't make the reads or they just couldn't keep up.
Because the Penguins play this way, their wingers must be consistent forecheckers. Pushing back against opposing attacks by utilizing intense pressure is how you slow them down when you're not big and physical.
The wingers you've noted as struggling and who have gone on to have some success elsewhere all have a significant deficiency in their games. Kessel is a known mercurial player who avoided contact out of self-preservation. He focused solely on offense, which is fine when he's tearing it up.
Galchenyuk has chased consistency in effort and attention to detail his entire career. It's why his career has absolutely cratered at age 27.
Pearson lacked speed and doesn't have the highest hockey IQ. It's why he's had more success in slower, heavier teams.
Zucker's only issue is his health and production. He's actually been fine, otherwise, and Sullivan has said as much. Zucker is smart, he reads plays well, and he's aggressive on the forecheck. He just can't seem to score and he's now battling a string of injuries.
Kapanen is Galchenyuk. The talent is there. The attention to detail is not. When Kappy is engaged and on-point, he's excellent. When he's frustrated or out of sorts, his game drops significantly.
None of these issues are because of Sullivan. It's who these players are. And all of them have shown these traits with other teams, not just the Pens.