We could get multiple pieces, but not ones that have the same impact as Marner, even cumulatively, unless we fluke into some player or future unexpectedly exploding in quality, which is not something you should rely on when making franchise-altering moves. If we were getting somebody of similar age and quality, then I'd be listening, but very, very few of those players exist in the league, and they aren't traded. This is why nobody who suggests this actually has a real proposal.
I don't think we want to send the message that we're incompetent and make the same old mistakes that destroyed this franchise for decades. It is about winning. That's why you keep one of the best young players in the world.