The data that LilRip provided shows that the top 4 scorers on these good teams range from 57 to 83 with an average of 75 pts. I think for this roster, the top 4 scorers should be on the upper end of the range so say 78 pts/gm. The top 4 for next season are expected to be Kemba (25.6 pts last season), Tatum (15.7), Brown (13.0) and Hayward (11.5) and totaled about 66 pts/game last season. (Actually, the top four the Celtics were Irving, Hayward, Morris, Horford and totaled 67 pts)
By my analysis, that means there are 12 more points for these top 4, not less. I hope that Kemba comes down, call him 20 pts although I think he could be very good on very efficient and be more like 18 or 19. That leaves 58 pts/gm for the other 3 (19 each). I don't see the problem, even if Kemba is more like 22 or 24 and the other 3 are in the 18 range.
The individual averages are skewed because nobody plays 82 games. The individual averages end up being higher and you really can't think of them as in aggregate. For example, last season the top 10 Celtics players averaged 122.5 in aggregate. The total of the individual averages will not equal the average per game for the team, unless every player plays exactly 82 games. Each time a player plays less than 82, it skews things. But looking at the teams from history, you can see what a good target is, even if it seems high.