Yeah, Cooper really doesn't like giving guys that kind of shot until they prove something, usually for a long time, on the lower lines. Or until his hand is forced. Could it be something coming from the players? Like how we have seen Kucherov complain about talent being removed from his line.
It seems he puts anyone with who can slightly "create offense" with lesser players in hopes that he gets some offense out of that line though it rarely happens. Not everyone is a Kucherov who can create space for everybody, lots of offensive talents need other offensive talents to bounce off of and they don't get that chance here despite the top end players we have.
JAM, CV all showed flashes and never made it past a couple shifts on the 3rd line, Drouin was primed to play with Stamkos and got stuck with Boyle and Morrow. Miller was a PPG player on the top line, Cooper plays him on 4th line for "extra scoring" and he has his worst season in 4 years, then leaves and becomes a PPG+ player again on a another top line.
ABB with Stamkos makes too much sense but we stick Nick Paul and Killorn on there? The guy is on his last chances in the league, the least we can do is see if there's anything at all worth salvaging.