Well, this thread is a shit show, with everyone thinking that their scapegoat is the reason this team fell apart. Not that I fault any of you for venting -- by all means, vent on.
I have my own theories about the problem -- namely, that we lost offensive depth in the off-season because of an unexpectedly flat cap and the TJ Brodie acquisition to (successfully) round out our defence. That left us as a one-line team when Tavares went down, which should have been fine, because that line had two elite scorers on it. Except those scorers didn't show, leaving us as a zero-line team with only a few individuals (Willie and Spezza) able to create offence individually.
Therefore the bulk of the blame has to fall on Matthews and Marner, IMHO. But there is plenty to go around. Dubas' trade for Foligno is going to go down as one of the worst in recent Leafs history. Keefe's inability to fix the PP, dropping Dermott for Sandin, and refusal to put Nylander in Marner's spot are just some of his questionable decisions. Engvall and Hyman didn't bring the jump they are capable of. Thornton was selfish to continue hogging a spot in the lineup despite being trash (if he was okay with sitting, he probably sits, no?). Simmonds disappeared after the first few games. Guys like Dermott, Sandin, and Galchenyuk made game-losing mistakes, although at least they seemed to put in an effort.
I honestly don't know what the solution is and I think emotions are too raw right now to make any sound judgment on what's next. Whatever trades are made actually need to make this team better -- a tall task if a player the calibre of Marner or Nylander are the ones heading out.