The Duck make this trade for the sole purpose of bolstering their future roster. Wahlstrom joins a young offensive group missing a big bodied offensive presence on the right wing to go along with Comtois on the left wing. They also bolster their defensive depth which might be an issue as some of their current defense exit their primes. In exchange for gaining two extremely talented prospects the Ducks also agree to take on the Ladd and Boychuck contracts. This works for the team because higher end players like Zegras and Raymond (this is a lineup based on the Ducks tanking and getting a top three pick in the 2020 draft) won't need contract until 3 years anyways. Plus this roster has so many ELC and cheaper contracts that the team is still left with 10+ million in cap space. The Ducks also give up a second because of the length of the Fowler deal in which the back half of the contract may pose problems for the Islanders.

Rakell is still only 26 and has two more years on an extremely team friendly deal. He immediately provides valuable secondary scoring behind the likes of Barzal and Lee.
Fowler is only 27 and despite some recent injury plagued years on a better team he could quarterback a power-play and put up 30+ points.
Sprong is 22 and still has some upwards potential, with the right line mates he could have a 30-40 point season and provides cheap, young and fast secondary scoring over older options like Komerov.
Overall the Islanders don't get considerably older, and they also don't take on any cap, in fact for the next two years they will be save approximately 1.613 million a year. However, whats most important is that the Islanders add nearly 100 points combined with Rakell, Fowler and Sprong. and none of the players is older than 27. The other benefit of the trade for the Islanders is that they unload the Ladd contract in which they are paying him 5.5 million to be a healthy scratch. They also unload the last 2 years of Boychuck's contract. This is another cumbersome contract that might poses a challenge when they try to fit Barzal under the cap.