So you're saying that when Monahan plays with 3rd line players he produces like a 3rd line player?
There is certainly some level of inefficiency you can assume in their time away from each other (end of shifts, adjusting to new linemates etc), but that doesn't come close to explaining the variance for Monahan.
For reference, Gaudreau's P/60 goes from 2.36 with Monahan to 1.98 without Monahan, a decent drop but pretty easily explained away by the factors above. And a lot of that time was when Josh Jooris and Markus Granlund were his linemates.
Who's to say? Monahan has not had a consistent linemate outside of Gaudreau in five seasons, and the times he has been away from him for any extended period of time were with players who were out of the NHL within two or three seasons. It's not a fair assessment to make in my estimation.
Monahan has the ability to play with and augment high end talent. He is not a third liner, and if he is being used as such he is being horribly misused. I agree it is now time to split those two up as I feel they have both become to reliant on what made them so successful for so long.
Gaudreau can better compliment lower end skill players because his game is reliant on having the puck on his stick, and creating offense for himself and others through transition and open space. That doesn't make Monahan a poor player for having to rely on smarts, positioning, and a more cerebral approach.
Monahan can and would produce with just about any legitimate top six forward in the NHL. Is he better suited for a 2C spot on a contender? Absolutely. He's not, nor should anytime soon be a third liner however.
Monahan is not a product of Johhny. They are both very talented players that have helped one another, and would find success elsewhere if traded.