The reason nobody would say that is because very clearly, none of them are, and it's a ridiculous thing to say. You attempted to justify your poor position on Marner by pointing to the playoff production dip that you ignore the actual reasons for, but even the best, toughest cup-winners have had the same thing happen to them, and you even helped show how situation and team-based playoff production is, so now you've admitted that the playoff production dip doesn't mean that. Which means you have nothing.
We did have around 20m cap space. Obviously, that doesn't mean 20m with a full roster signed, just as it doesn't in your scenario.
There's really no combination of available options that necessitates losing Marner, that is worth losing Marner.