That he has stalled is not really true. This season he went from playing a sheltered role to playing a secondary match up role with much, much tougher minutes. Despite that he saw his point production at even strength rise by quite a bit*, and the only reason he didn't hit 70 points this season is because of the PP. Analysis of the second PP unit shows that it wasn't the fault of Willie or Auston**, teams just took away the pass between them which left the puck carrier as the only threat as none of the others did anything at all, ever.
He was definitely invisible in the playoffs.
One thing I don't understand here is the idea that due to the idea that he's stalled, he isn't worth committing too long term. That's absurd. Even if he never would have developed going forward, he's still a first line forward with elite transition game. You tie up a player like that long-term in an instant. And that's in a scenario where a guy entering his 22 year old season never develop.
* He ended up at #44 in even strength production in the league, despite playing less than almost everyone above him.
** Speaking of Auston, I know people will bring him up as a retort to this. When apart, Auston suffered more than Nylander. Willie went on to play easier minutes but with worse players***, and actually improved in many metrics. Not the same production, but that's quite understandable given the difference in shooting talent on his line. Auston on the other hand fell off a cliff when Brown took Nylander's place. Which is not said to try and diminish Auston, it's just very hard to get much done playing with two guys that have limited offensive and transition ability. But Willy takes on a huge role on his line, all the way from winning pucks back**** to transitioning the puck up, to taking on the main playmaking role on his line.)
*** And we know that linemate quality has a bigger effect than opposition quality.
**** Willy ended up as a top 5 player in the league when looking at venue-adjusted takeaways compared to ice time.