Somewhere along the line, the team became about the big guns, rather than the team as a whole. It's not really about one of them individually, but rather how as a group of four, the team revolves around them. Their ego's are of paramount importance, their wallets our biggest priority. It seems like the team's identity has become "our best players are better than your best players" and that's the be-all, end-all.
I think it stems back to the contract saga. Nylander holding out was a disaster, the JT contract set the bar high, and then Matthews and Marner both fought to cash in. Believing that these guys were talented enough to lead us to the promised land made paying them seem like a worthwhile gamble, but as a group they just can't get it done. From here, the organization let things spiral out of control, naming Tavares as Captain was a disaster that ensured a weak dressing room, and getting a butterball coach in Keefe meant even weaker leadership. A few years of this, kowtowing to the players despite their failures come playoffs, compounded into a team culture that's summarized as "we're still hot shit guys, we'll get 'em next year". Their egos match their paychecks, not their results.
If I had to pick one specific moment here, I'd say that the Tavares signing was the mistake. Partially because it put far too much of our cap structure into forwards, partially because it blew up our internal cap structure, and partially because it lead to him as our captain and his weak leadership has allowed the team culture to snowball. Marner is the biggest problem we have today, but I don't think he turns into this horrendous of a playoff dud with better leadership and a different salary cap narrative.