no, they should have put Tatum in the exact same role, and subbed him in 1:1 with James whenever James went to the bench. 25 mins a game for Lebron, 15 for Tatum, taking advantage of their super-well-rounded skillsets, would've let everyone keep the same expectations no matter which one of them was out there.
Instead we got 5 minutes of (what looked to me like) "welp, looks like Tatum can't defend that center in the post!" after which Kerr decided "Better staple his ass to the bench!"
Tatum's minutes in the tournament:
vs SRB: DNP-CD
vs SSD: 17 min
vs PUR: 23 min
vs BRA (QF): 20 min
vs SRB (SF): DNP-CD
vs FRA (F): 11 min
So he was a minutes-eater against Puerto Rico and South Sudan, but otherwise basically didn't play any crunch-time possessions for the team against legit opponents. Clearly, Kerr said "Tatum's a bad matchup against Serbia!" and instead of thinking "...so how do we get value out of him in this matchup? WWCJMD?", just threw in the towel and went with a default of "don't play him". Like, I'm not even sure Tatum was in fact a bad matchup against Serbia - he has the size to guard the wings and the agility to get out against the perimeter shooters... but suppose we spot Kerr that one. OK, so he's a bad matchup, maybe we think they're not going to respect his shooting or something. But he's one of the most versatile stars in the NBA, who has molded his game to do whatever is needed by his team over the last 8 years, playing a million different roles... and your answer as a coach is "man, I got nothin', I guess we just can't use him at all!". Doesn't that say more about the coach than about the player?