I think DeRozan fits in other systems, and other teams will value him as such.
The problem with his fit in San Antonio boils down to two factors: LaMarcus Aldridge and poor defensive talent.
First, LaMarcus and DeRozan both play antiquated, mid-range ball...a team can survive one such player who thrives that way, but when your two highest usage players do, it's hard to survive in this three-happy version of the NBA.
Secondly, the team just has too many sub-par defenders to hide DeRozan's own sub-par defense. It's why we are sitting at the bottom of the barrel in basically every defensive category right now.
Simply put, the makeup of the team wasn't really built for DeMar when he came in, and we've seen the results. That said, I still don't think he is worth keeping and trying to build the team around him.
The biggest problem I see with trading him is his player option, so you have to find a suitor that is either looking for a rental, an expiring contract, or believes he will stick around. That, it itself, is going to hurt his already tainted trade value.