Unless you are a super team like Golden State, NBA games tend to be close, due to the team that is losing trying harder while the team that is winning letting up. That is why you need guys like Smart who can make big plays on a regular basis. There's no such thing (unless you are obscenely great like the Warriors) as adding a better shooter (but lesser overall player) and suddenly blowing teams out every game so that you never need heroics. Heroics are ROUTINELY NEEDED in the reality of most NBA contests...
Irving plus Hawyard plus Tatum plus Brown plus Horford is MORE THAN ENOUGH OFFENSE for one team. There is only so much offense to go around. You keep Smart on the team if you're a smart GM. Of course, affordability does come into play which is somewhat out of the Celts hands right now...