My only problem with the story so far is that at no point was Walt really a kingpin. He was just a cook who sold a large batch of stuff to people from time to time.
He never really had an army of subordinates, never really dealed with any cartels on more than one-time type deals, and the only people who ever really feared him were the people who saw how batshit nuts he'd get and hire people -- again, only temporarily -- to do the real work for him.
That's not really a "kingpin" to me. Not even close.
The way I see it, he was a kingpin for all of two episodes last year. Once he made the deal with Declan, he had reached his mountaintop. Since the show does not have 20 years to show a gradual fall, he retired immediately.