When Bert, Mantha and Larkin were hot they were a special special line. Sadly Mantha without the other two looks pretty mediocre, and Bert just can't stay fit. Even without the extras, Copp and Compher are useful multi-position guys who probably retain decent value as assets, and will likely generate more picks come their last contract years.
Also, the reality is, that Copp and Compher have scored 180 combined points on last 2 seasons, when Mantha and Bert have 142 combined points.
142p/222games = 0..639 ppg
180p/306 games = 0.588 ppg
Mantha and Bert have been slightly better scorers per game (mostly thanks to Bert), but they aren't keeping healthy. Copp and Compher can complete 82-game seasons, which is a pretty big thing at success. They have also the bigger defensive duty as Centers.
Mantha had abysmal last season, scoring only 27 points at 67 games. A guy like Daniel Sprong (very one dimensional winger) just almost doubled that scoring.
Mantha, 67 games, 11+16=27 points, -8
Sprong, 66 games, 21+25=46 points, +13
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I really have to admire, how Yzerman saw/planned this big picture how to evolve the situation and fix the center core. Trade these winger assets, when they still had value. He did transfer the value on another positions. Not especially on centers. Wingers, picks, defence, prospects. And on two free agencies, he was able so fill those needed Center slots with good options (2nd best quality Center on both free agencies, Copp after Kadri and Compher after ROR)), which are still, imo, pretty young, and we'll get their final prime years, until they are 32 year old.
Now those guys are in here, and at same time we have finally those new kid centers drafted in the systems. They are not ready or pressured to take any kind of torch for this team, but will be in few years. You kind of see the timelines, Kasper and Danielson ELCs are the growing years for them and Copp and Compher, behind 1st star Larkin, are the Center backbone for this team, before the kids are really ready to take the torch.