The Leafs failures were compounded in a number of ways. I'd sum them up as such:
1. Deciding that Rielly was a legit #1 (he isn't even close) and then compounding that issue by getting rid of any other decent puckmovers on the blueline.
2. Signing Tavares to a deal that got, at best, two prime years out of him.
3. Signing him to that deal at the exact same time their three most important core players were up for contracts.
4. Not firing Mike Babcock the very moment he pulled that shit with Marner.
5. Not getting good enough goaltending.
The rest basically comes down to them chipping away at their identity (which Treliving continues to do) until they were a big slow collection of grinders outside of the "Core Four." I think they could have been successful even with Marner, of whom I'm not a big fan, but they just went about it in all the wrong ways.