I would take a slightly different direction. Instead of comparing to past team's, compare to current events. The Leafs are a top 5 offence in the NHL with a good prospect pool that is still contributing this season and likely next and this is without the big 3 playing huge minutes like most elite players do (a late proponent in why they score so much in the top 10 is how much more they play than Matthews and co. and they get stacked PP time). By 2020, the big 3 will have to play more as the new ELC guys will be coming in. Andersen is a 920+ goalie and Rielly has taken a huge step this year.
See it like this; by 2020 we have this rough core.
xxx-Matthews-Marner
xxx-Nylander-Kapanen
xxx-Kadri-Brown
xxx-Aaltonen-Brooks
Then fill in the X's with guys signed/Marlies (Korshkov and Johnsson are my bets for the first two lines based on style. Hyman on a Kadri-Brown line sounds insane considering all the forechecking potential. Fourth line is a toss-up.)
Rielly - Hainsey/future Hainsey replacement
Dermott - Liljegren (They have looked amazing together in the AHL)
Gardiner - Zaitsev (or Gardiner replacement at this point)