I do find it interesting that the one person in the pentagram cult who is alive at the end of the game is Michael Bains. I feel like that was done on purpose, that they were going to bring up back for PQ4 just like they brought Jesse Bains back for PQ2. But then of course Jim Walls left Sierra and that changed everything, and I guess his new company didn't have the rights to Sonny Bonds and all those characters. Makes you wonder who did? Since Daryl Gates' PQ4 is entirely new characters, it seems Sierra didn't have the rights to Sonny Bonds and other PQ characters either.

I could be wrong but, even if Wikipedia says his departure from the company happened late into the game production, I recall reading that Jim already had a foot out of the door at the very beginning of the game production. I think he provided general ideas and guidelines at the start but was not present during a big chunk of the production and the game was mostly designed and written by Mark Crowe and Jane Jensen as a result. I don't think Jim had plans for a fourth game. If anything, it's probably Mark or Jane who left that door open in case Sierra wanted something to build a potential sequel from. I do recall that he had plans for a follow up to Blue Force however but that just didn't happen.

I'm not a big fan of the third game myself. It's okay but it's nowhere as good as the first two games, it lacks the whimsy charm that Walls put in the previous games, and plot wise it feels like a patchwork of all the gritty Hollywood crime stories tropes at that time.

I'm fairly sure Sierra still had the rights to Sonny Bonds and its characters, but Darryl F Gates didn't like the previous games, found them too focused on Hollywood type of stories and just wanted to tell a story of his own and give his vision of police work rather than follow up somebody else's story and vision.