Probably spoilers down here!
So here we are in these weird times and our lawyer is coming back with new stories to tell. This prequel is about the human creation of being the animal of mistakes and being involved in conflict into much stronger things than "I'm just the lawyer". We can all identify as a person, but not as a free will complex creation, that has control over people that we are all around. We know what will happen. We know how the stories of Gus, Mike, Hank, and Salamanca Family will end. We just don't know how. How Lalo S. and Nacho end, what is the fate of Saul and Kim? And this is the main question that we want to know. It's not about who they are, but what is the process of these characters. Now, what we know: Saul is the creation of a few big things and other complex things: Jimmy wants to be a charismatic lawyer, be the god of the law, but the death of his brother, then complex misunderstood between his identity and what Kim what from him, creating a wheel that going to the dark mood lake. Another ultimatum of these things is the fact, that Jimmy creates a new person, "Saul Goodman" to be a marketing lawyer-machine (which we saw at the end of s4).
Step into the wild is the nihilism of the situation in the relation between Jimmy and Kim, which creates... well, marriage because they want to feel that they have control of the situation. Jimmy wants to not confront the problem but to shortcut the way that he must confront it. And this is the fatal death of his persona, which makes him a nice guy to Lalo Salamanca. And we have here breakdown, the will of life to come back to Kim, and the desert event with Mike, this gave Jimmy a moment, that he now understands, that he has to control nothing. And really want to change it, by lying to Lalo and Kim about what happened in the desert. In the final, Lalo survives his assassination and wants to get revenge on probably Gus, 'cuz he knew it was Gus with Nacho. Probably this will be the totalisator to Kim losing his identity and then Jimmy became absolutely Saul Goodman in a mental way.
The Fantastic World of Jimmy McGill and ItSaul'Goodman tragic comedy
The gray scenes of the future of Saul, after the BrBa, show us that he really needs to confront his guilt after these years. And the guilt is all the lies web that he created to feel a distance from the feeling of being men with mistakes. The emotional status Jimmy creates Saul and breaks the status quo about what he thought about himself in the past (BCS). And it might be his other doom. Just like Kim going down because of his identity, just like Hector goes down because of confidence and not seeing sneaky tricks from Gus and Nacho.
Just like Nacho might fall because he finally can feel control and just like Don Eladio and Gus because they all think that there are no men who can break their web of control (and this doom have the name of own frustration and loss, named Walter White).
Because pretending is only the mask of pain for the lost identity. Saul and Kim are the only other avatars of false memories, avatars... with their own emotions.