Zadina always had the talent to have an impact here, but came up too soon and struggled to swim against the current. There were plenty of times over the last 2 years where he looked great up until the final action - which always suggested he'd improve if he could find confidence/feel more comfortable...its why Yzerman renewed him and clearly wanted to give him another chance, but Zadina took matters into his own hands.
I'm sure he'll do ok on a SJ team desperate for any skill up front to play with their only talent - Zadina has always looked decent when played with the handful of guys he had chemistry with, so if he can find that elsewhere, he'll make us feel slightly sore about his departure. Ultimately what did for Zadina here was Vrana going off the rails, as the two meshed well.
Management can't really legislate for one winger scoring at an elite pace going into either therapy or rehab, and another who underperfomed refusing to report. Its hugely disappointing as Vrana on 3rd line (and making Zadina useful) would be a massive boost for this roster and allow different resource allocation, but it is what it is. These guys are all human. The only comfort is Mantha has been trash in Washington, and there was only 1 F taken in 2018 after Zadina who hasn't also hugely disappointed - and if we'd drafted one of those D we might well not have got Seider