My views on assists is different. To me, team assists are more important than individual assists. I don't think the passing ability of the PG is any more important than the passing ability of any other player. If you look at the top teams for team assists last season:
GSW
DEN
SAC
PHX
SAN
IND
BOS
Many of these teams had a none PG as their top assist maker:
Green
Jokic
Sabonis
Paul
Tre Jones
Haliburton
Smart
Passing is not just about the PG. The best shots come from multiple passes, not from a PG dribbling around and making 1 pass. The Celtics should focus on offensive balance, off ball player movement, and of course, moving the ball (aka scheme). The Celtics were 7th in the league in assists last season, with what most everyone agrees was kind of a stagnant, inefficient offensive scheme. This isn't even a problem so there is no reason to something major to try and fix it. You don't need a ball dominant PG to make the scheme better, to get more off ball movement, more cutting. Scheme is about coaching and about 5 players being connected.