The key is that they've signed him to a three year deal. If it had have been a one year deal as initially speculated it would have been a disaster as he would have had one eye on the Roosters.
I still feel like it's a square peg into a round hole type of signing. What made Valentine Holmes such a special player was his ability to make plays deep inside his own half. Whether it was picking up a grubber on the half volley and racing 15m or catching the markers out from dummy half, Holmes provided the Sharks with a real edge. Presumably if Moylan moves to 1, the Sharks will lose that, making it difficult for Johnson to even involve himself in the contest.
They could move Dugan to 1, but it remains to be seen who will play in the centres. With Leutele, Ramien gone, it leaves Josh Morris, Kurt Capewell and Aaron Gray as the only viable options. At this stage of his career, Capewell is better in the backrow and Gray has yet to prove himself in first grade. They'll have to hope that Bronson Xerri and Jackson Ferris come of age sooner than they would have hoped. It also means that somebody between Moylan, Townsend and Johnson will miss out, making it an unrealistic option.
On the plus side, we haven't really seen what Johnson can do behind a strong pack before. Playing for NZ, Johnson always appeared to be a much better player when he had Bromwich, Taumalolo, Taupau etc. to play off of. With Fifita, Woods, Graham, Gallen etc. there he'll walk into a better pack than what he had at the Warriors and will have more opportunities to play off of second phase. If he can make the most of the limited opportunities, he could easily cement himself among the game's elite, but it's a really tough ask. There will be games where Cronulla may only get six shots at scoring, if the team doesn't deliver, all the blame will be on Johnson.