After KD went out last year, Warriors still beat Houston and Portland to get to the Finals with no real 3rd option behind Steph/Klay.
Cavs won the Finals in 2016 with Love giving 8ppg and made it to 6 in 2015 with Love and Kyrie both out and no real 2nd scorer, much less 3rd scorer.
Spurs in 2013 and 2014 didn't have any big scorers like LeBron and AD and still made it to the Finals with Kawhi's 12-13ppg as their 3rd option.
Mavs in 2011 had Shawn Marion's 12.5ppg as their 3rd option.
Lakers in 2009 won with Bynum's 14ppg at their 3rd option.
And the thing is, you can say scoring is higher now but you can also say that none of those teams except the 2016 Cavs had a 1-2 scoring punch as strong as Lebron and AD. Lebron+AD+AB gives you as much scoring as any of those teams had, and when you throw in Danny, Dwight, KCP, and say a Covington/Morris/Bogdanovic that they pick up with a Kuzma trade (or Collison as a free agent), that's plenty of points to go around. When it comes down to crunch time, you're going to be going with Lebron, AD, kickouts to 3pt shooters and opportune off-ball cuts. You don't need 3 scoring threats like that.