"Pre-Bylsma" should be "Pre-Eichel and O'Reilly". His drop in production had less to do with the head coach, and more to do with better players ahead of him.
Zemgus was the first line C in 2014-15, he played the power play, etc.. and saw over 19min/gm of ice time playing with the top offense players. He averaged 0.5 ppg that season.
Under Bylsma he was relegated to 3rd line minutes (which is where his skill set belongs IMO). His time on the PP dropped because of ROR, Eichel, Reinhart, Kane, Okposo, etc.. NOT because of Bylsma.
Zemgus has the skillset to play 4th line minutes on a contender, maybe 3rd line.
To blame Bylsma for his drop off in production is a joke.
Why not both? Zemgus scored less because he got less playing time, and also because the coach was trying to coach with 17 whiteboards. I think Zemgus is a guy that plays with his instincts, and Bylsma was crushing his hockey soul.