The Islanders really should've rebuilt when Tavares left. They brought in Lamoriello in 2018, he helped bring them out of perpetual mediocrity but he pretty much doubled down on bolstering a middling roster. Back then even at that time I feel they missed their opportunity to do a proper rebuild/retool, and old man Lamoriello was never going to do that. That's what probably should've been done but I'm sure there was pressure to get back into the post-season and improve the optics of the organization.
When he took over in 2018 they were coming off 2 playoff misses in a row after seemingly a never ending run of little to no playoff success. Tavares left that summer as a free agent, once that happened they should've tore it down. Brock Nelson and Anders Lee needed new deals the following summer. They had too much money tied up in the bottom 6 and Andrew Ladd's albatross of a deal eating +5 million a year to be a bottom 6 player was the icing on the cake. Johnny Boychuk was a shell of himself making 6 mil per.
But Lou just kept doubling down on that roster bandaid after bandaid. After Lee signed that albatross of a contract the following summer he's only had one 50 point season since. He's never lived up to his 7 million a year price tag. Eberle and Bailey were good trade chips at the time. Varlamov could've been a chip when Sorokin arrived. Leddy was the only vet piece they winded up selling. They gave up Devon Toews to Colorado and then 2 years later ended up trading the 13th overall pick for Romanov to replace what they missed out on. It's not like they were completely devoid of young talent either. It could've been a quick turn around. Guys like Barzal, Toews, and Pulock were just entering the league. They just drafted future #1D Noah Dobson. Sorokin was ripping the KHL. They had future core pieces in place. They did really well building through the draft then they changed course.
Barry Trotz dragged those teams into the playoffs. And they had some good success too. But it was always a team you'd look at and say, yeah they're decent enough to grab a playoff spot but they're missing the high end talent to be a real contender. They had some good runs. But that's what it looks like Lou has built there, a team that's perpetually solid enough to compete for a playoff spot but lacks the star power to win a championship. They wanted Panarin, they tried to get Johnny Hockey. That's the only way they're going to land those players since they've traded all their picks away. Not a great formula to pray a star free agent comes to save you
But I suppose Lou served his purpose though, they made the playoffs 5 of the last 6 seasons. Had some deep runs. They had all that arena drama, now they're settled into a more permanent arena. The surrounding area is being built up around it as a destination. So in those aspects it was a success for the organization. But in terms of building a future cup contender, not so much.