Their core is okay, Marner, Matthews, Nylander, and Rielly are great but Tavares is pointless. The Matthews-Tavares 1C-2C combo is a failure and it reminds me of San Jose annihilating their cap structure to pair Burns and Karlsson together, turns out they get in each other's way and 90% of Burns' points came from Pavelski tips. Look at the successful teams of the last decade and they are a lot deeper than Toronto, Hawks had Toews, Kane, Keith, Crawford, Seabrook, Hossa, Sharp, even Byfuglien was there for a bit. Kings had Quick, Kopitar, Richards, Brown, Doughty, Martinez, Muzzin. Penguins had Letang, Crosby, Fleury, Malkin, Kessel, Guentzel.
I don't think they have a very good team, but they have an amazing four players that just aren't enough to win.