Why the San Antonio Spurs may consider bumping Poeltl back to the bench
While Poeltl is certainly a good starting center, I'd be lying by saying that he isn't replaceable. If San Antonio can find a better option than Poeltl, then they shouldn't think twice about sending him back to the bench.
Because Poeltl only shoots about 55% from the free-throw line and doesn't have a reliable jump shot, his offensive ceiling is severely limited. To Poeltl's credit, though, he's quite efficient in the shots that he does take, converting on an amazing 61.6% of his them, though it's unlikely that he gets much better than he already is.
If the Spurs were to find a replacement for him in the starting lineup, it would likely be with a far superior offensive player. That said, that player would also have to be at least passable on defense to avoid making a lateral move.
There don't appear to be any undeniably better upgrades available in free agency that fit that description, though there may be long-term upgrades in the draft. Players like Alperen Sengun, Isaiah Jackson, and Kai Jones are all prospects with more upside that could be available with the 12th pick in the draft. However, while each has the potential to grow into a more impactful player than Poeltl, there is no guarantee that any of them reach that potential.
Sengun could certainly prove to be a much better offensive player than Poeltl, though he projects to be an average or below-average defender. On the other hand, Jackson and Jones could both develop into better defenders compared to Poeltl, but projecting their offense is a bit difficult at this point.
Ultimately, though, replacing Poeltl in the starting lineup is something that should at least be considered. Because there aren't any clear-cut upgrades on the roster, in the draft, or in free agency, such a move would certainly involve some risk, but the reward could be worth the gamble.