They need to learn at some point. Secondly, I'd rather have more naturally offensive players in that slot.
Wouldn't you rather Zacha prove that he is absolutely ready and start to push Zajac down the line-up? To me, I'd rather keep guys like Greene and Zajac in their roles to protect the kids who aren't ready. Make the kids improve their games to the point where they are ready to play the role and ease the veteran players out of their roles.
That's how Parise earned his minutes. That's how Greene earned his minutes. That's how Zajac became our top center with Parise. That's how Gomez earned his spot on the Devils early in his career. Look at Rolston/Guerin/Niedermayer/etc. in the Devils first cup run. If you just throw kids into roles they aren't ready for and hope they can pull it off is where you get into the mess that Edmonton was before they lucked into McDavid.
Devils were successful for a long time by keeping the veterans until the kids made them expendable or pushed them into lower roles. Granted those veterans where better than Greene/Zajac, but I'd rather try to get back to that earn your minutes system we had.
At this point I really do not care. This team needs to trade valuable players to get better for the future, not hold onto these players with some value just because a rookie or a coasting player would be taking over their minutes. The team sucks with them, might as well get some value from them and try to get better for the future.
This mentality is what partially got us into this unstructured mess of a team we're seeing right now.
We let all the veteran walk that played a sound game. They sucked offensively and their best years were behind them, but they knew how to play their position and play well defensively. Now we're left with kids who clearly aren't ready and/or don't do the little things right to play structurally well and no veterans to play those minutes or to set the right example for the kids.