Galchenyuk's goal totals were on a rise in each of his first 4 seasons, climaxing at 30 goals in 2015-2016.
He had especially found a sweet spot on the powerplay, but I think he had improved reasonably well in his first four seasons anyways. Off ice issues added another dimension to his troubles though.
Here's his 5 on 5 numbers when adjusting for ice time
12-13: 1.03 goals and 2.85 points per 60 minutes with 12:19 ES per game
13-14: 0.62 goals and 1.4 points per 60 minutes with 14:24 ES per game
14-15: 0.84 goals and 1.8 points per 60 minutes with 16:26 ES per game
15-16: 1.09 goals and 2.07 points per 60 minutes with 16:16 ES per game
Then looking at PP numbers when adjusting for ice time
12-13: 0.0 goals and 0.0 points per 60 minutes with 1:03 of PP time per game
13-14: 1.36 goals and 4.53 points per 60 minutes with 2:02 of PP time per game
14-15: 1.01 goals and 4.05 points per 60 minutes with 2:13 of PP time per game
15-16: 2.57 goals and 4.58 points per 60 minutes with 2:33 of PP time per game
When I look at those numbers I don't see much progress over the first 4 years, after his rookie year he basically spent 3 years just getting back to those rookie numbers.
And it's also worth noting that as his ice time went up it means he played more frequently with the teams better players like Pacioretty. The story was pretty much always the same, he'd get promoted to play with Pacioretty and his scoring would take off, then the line would be broken up for reasons only Therrien knows. So in many ways he wasn't actually progressing as a player over those first 4 years he was just spending more or less time in the top-6.