We probably do have more talent on the perimeter than on the interior, but it isn't a zero sum game. And as far as depth is concerned, it remains to be seen how much of it we have in the back court. As of now, the players that the addition of Napier would have the largest impact on would be Samuel and Cassell, and although I think those two can be good players later in the year, reducing their minutes isn't the worst of things. Purvis and Hamilton are capable of logging enough minutes at the three and four, respectively, to contribute meaningfully either way - essentially, you're just downsizing and sliding Facey out of the starting lineup. Hamilton can function as a stretch four on offense and hold his own defensively with some help.
I understand your point, and it is not a zero sum game, but our weakest starter and weakest bench spot are both at the 4 (Lubin just isn't ready). By adding a star at that position and pushing Facey to the bench, we're also increasing our depth in the area we're weakest
On offense, there's only so much dribbling that can happen. You need versatility of skills and roles to be truly lethal. We won the title with Napier and Boatright last year, but we also had an inside-outside space-opening threat in Daniels (we won every game he scored 20+ points and he averaged 2 points less in losses) and a 3 who didn't need the ball to make an impact on offense (Giffey/spacing). Purvis at the 3 is just further reproduction of similar roles (and without an effective 3 point shot he's a waste of floor space alongside 2 other penetrators). I also have serious questions about Hamilton guarding a 4.
Not to mention Marshall may just be better than Napier. He averaged more points from a position that is harder to score a lot of points from. He was a 15-year NBA player for a reason. He might not bring the same leadership intangibles, but I think Boatright fills a lot of those same needs.
PG Shabazz \ Cassell Jr
SG Boatright \ Samuel
SF Purvis \ Hamilton
PF Facey \ Hamilton
C Brimah \ Nolan
PG Boatright \ Cassell Jr
SG Purvis \ Samuel
SF Hamilton \ Donyell
PF Donyell \ Facey
C Brimah \ Nolan