He's not good defensively, but he WAS the regular 2B in 2023. I don't expect Polanco to be healthier or better at defense in 2024 so I don't really see him as a challenge to Julien as the 2024 starter. Polanco would be a $10M backup infielder who can be expected to play 80 games.
Of course, none of this has anything to do with replacing Sonny Gray unless you're talking about reallocating Polanco's $10M toward a starting pitcher.