Tavares wasn't the player the Leafs needed, but he's the player they got.
In retrospect, the Tavares signing was, and is, redundant to the Leafs. I'm not saying it's a bad signing, as any team in the league that could have afforded that contract would love to have him, but the Leafs are one of the few teams that didn't outright need him. He wasn't the missing piece to the puzzle, or some signing to replace a lost player; he's simply a very good player on a team that, even then, had obvious faults in it's composition that should have been the priority.
1 - Lack of dependable backup goaltending. Sparks and Hutchinson were both disasters and Kaski got straight merc'd his first game. it may not get better this season unless somehow the Leafs can swing for someone on the market who's cost controlled... which means they either ride and die with their current options or have to trade pieces to get someone.
2 - Defense. Yes yes, you got Muzzin. Problem is, defensively, Muzzin is all you really have. Ceci's a bum, Barrie is either improperly used or just plainly a bad fit for the Leafs. Marincin is Marincin. What other options do they have? Sandin and Liljigren? I don't know if throwing two rookies on this roster, no matter how good they look on the Marlies, is going to fix anything. More likely they get chewed up this season.
3 - "Grit". I hate seeing that. The Leafs don't so much need literal grit as much as role players that play with an edge. They need someone like a Blake Coleman or Kadri (when he's on his game)... someone who'll play a physical game but retain some skill and attention to defense. I have no idea how Kadri was defensively, but a Coleman-type on the Leafs would help them.
For the price of Tavares, 2 of the 3 above could have been resolved. Now, with the way the team is built (meaning, the cap being screwed), they have to move roster pieces to correct any of the above.
I do have to question Dubas on this signing... was he seriously trying to build a team around the HF joke "Outscore them by enough and you don't need a solid defense"? There again, how much of that contract was made by Lou before he left, and how much was actually Dubas?
Plus, if you believe some people, Tavares is the cause of Marner getting his point totals last year, so indirectly Tavares caused Marner's contract.
Even further down the rabbit hole for some people, Tavares caused Matthews' contract, too.