I think Poeltl is worth about the QO is the Spurs think he's going to be a backup for the length of the deal. If he can be a starter/30mpg guy, I wouldn't really mind $30M/4. Honestly, when the team is paying a guy $7 Million to not play, they can afford to give a guy who contributes a big more. Hopefully, once Pop leaves, the apparent disconnect between the FO and coaching staff won't be there anymore. Poe's a really good per-minute center, and I don't think that's really easy to find. For sure, definitely backups and third-stringers shouldn't get much money. But having a good defensive center will literally always be important.
I also think that people tend to think of spacing a one-dimensional, meaning just along the three-point line. In reality, spacing is three-dimensional, and having a rim-running big who finishes well will open things up for everyone else. You can't play five-out. Right now, DeRozan plays inside enough to where the team benefits from a stretch-five. But that may well not always be the case, and it already isn't with the bench. Poe's lack of shooting isn't close of the biggest problem he has. He has to focus on improving his strengths like being a more consistent rebounder and keeping his fouls down rather than getting new skills. There are a number of defensive C's who score opportunistically on good teams in the league today.