This is an astute observation and I agree that the team's shooting isn't getting enough attention (as opposed to its bigs).
By the way, Philly has an even bigger question mark when it comes to shooting. They retained Harris, but the loss of Reddick is going to be a bigger deal than people realize.
As for the Celtics, I think the solution needs to come in-house. That is, you hope that Smart, Tatum, and Brown continue to improve from the perimeter, that Hayward finds his stroke again, that Theis can hit the 3 now and again, that Grant Williams can extend his range, etc.