Islanders don’t really have the assets to acquire Pettersson.By all accounts, Barzal is happy with the Islanders and wants to spend his career there.
Value isn't far off from a player to player perspective per se, but one side is locked in long term and happy while the other is not locked in long term and wants to have the chance to compete for the Stanley Cup year in and year out.
There's a greater chance of the Islanders looking to trade for Pettersson using other assets or sign him on the open market than them considering swapping Barzal for Pettersson.