To me, BCS was a show that was a bit more like The Sopranos than Breaking Bad was. Breaking Bad was more operatic, dare I say a bit more 'cartoony' in places (honestly, the version of Saul we meet in BB is way more of a cartoon character than the person we get to know in this series), and all about tying up every loose end. The Sopranos was more about the subtlety and ambiguity, letting the audience fill in the gaps. And it was richer for it. BCS, to me, was kind of like a combination of both approaches.
I think BCS overall was more about the subtle moments, being more like-life, letting the audience make certain connections. Particularly when it came to Jimmy/Saul and Kim's story. Less so on the cartel side of things, which was more of a Breaking Bad prequel that was connecting dots.
I also think the second half of the finale season was more of an epilogue to both shows. I think Gus and Mike's stories were left in a place where everything connected to the characters we meet in Breaking Bad. We see the event that fully turns Saul into SAUL and drives him apart from Kim. So what's left was seeing Jimmy and Kim's ultimate fate. I think the final episodes delivered on that front. I too was left with a feeling of, "that's it?" when the credits rolled...but after sleeping on it, I honestly think that it overall was the right ending for the story they told in the show. I can't say it's an ending that I'm unhappy with.
Jimmy chose prison as his retirement. Even before he realized it-- I totally believe he wanted to get caught and that's why he was acting so sloppy. His life in Nebraska was ALREADY a prison for him. At heart, he's a survivor. The crooks he's locked up with are his people. They see him as their champion. And he's getting along just fine with them. So, in the end justice is served, he gets what he deserves, but there was still a piece of him that loved Kim and wanted to try and do one last thing to set things right with her and couldn't stomach the idea of her having her life ruined. If I can buy Walter White's one last act of "good", I can buy this. In the end, the guy who made a career out of getting lowlives out of jailtime ends up representing himself in court and doing the opposite for himself. I can appreciate the irony of that.
Sure, this wasn't a mind-blowing, "OMG I CAN'T BELIEVE IT!" bombshell sort of finale, but I think it was true to the show, which has always been a slow-burn.