If you have a 3rd line that can push the play, possess the puck and score, that’s different from a grinding line, but no less effective.
The initial question I asked still remains. Why can’t you have a 4th line that does a little bit of everything? Put three guys out there with different skill sets that compliment one another. And while I get the PK angle to rest the Top 6, many of the B’s young players are capable defensive players that can contribute more offensively than your traditional 4th liner
No one with zero offensive skill should be playing in the modern day NHL IMO. I haven't read the entire thread, but I also think the amount of skill you have in the bottom two lines is indicative of organizational depth. In a perfect world, someone like Debrusk is my prototypical "bottom six" player as of right now, given his skillset and development. Someone who goes to the net, responsible two way player, can provide secondary scoring and has the potential to move up in the lineup. Kuraly would be my prototypical 4th line center. Skillset to be successful there with the potential to move up.
For me, you incorporate as many young players as you can into the bottom six as soon as they've proven to be capable defensively (capable, not great). I think a veteran presence is necessary as well, but not the Yelles/Begins of the world (i.e. someone that can provide the leadership aspect without being a black hole offensively).
Definitely not a fool proof plan, but thats generally how I feel.