that was the whole point though, he was 2nd best lawyer in the land, he negotiated his sentence down to 7 and a half years, the whole thing with Kim was that she was absolutely done with him though. If he would have taken the 7 year deal then there is no way Kim would have ever been involved in his life. His whole life was worthless without her to share it with, he has no friends, he has no family, and I think the flashbacks to Walt and Chuck were brilliant because they foreshadowed what he was gonna do - instead of doing more Saul Goodman shit he owns up to EVERYTHING, does a full confession like Kim told him to, changes the course of his life like Chuck suggests, and even breaks the curse of Walt by doing something that is not selfish (oh so you were always like this).
It's the only way for salvation in his eyes, and if it means sitting in prison for the rest of his life, so fucking be it. Being on the run for that long warps your brain, once people are caught after a long time running the biggest feeling they get is relief because they don't have to worry about being caught anymore. Everything about Saul after being caught suggests that his main feeling is now relief, he can unload the emotional baggage and deal with the death of his brother, which he was directly responsible for, he can make amends to Kim, and he can now live his life without the stress of worrying about being caught. Ironically enough even though he is in prison he is finally free.