What do you base your judgment on? At the end of the season playoff run when Casey played on the top line, he played well The line not only didn't miss a beat without Tage but it excelled. With respect to Krebs, his solid play on the fourth line had no drop-off during that end of season pressurized run.
I fully expect Mitts to be a beast in the playoffs, especially if he's on the third line. I could see other teams' top lines neutralizing Tage's and Dylan's lines and the series being won from the third line.