Miles, we loved The Sopranos over here and with no disrespect to your opinion we thought that phase of the series was very well-done. Here was a mob boss who had been expanding his crews areas of influence and operation coming to realize that he was becoming too big of a threat for the authorities to ignore. He wisely decides that he needs to drop into the shadows and put a few layers of plausible deniability between him and the crimes.
He also wisely surmised that he was getting older and he needed to start grooming a replacement to head the crime family. What better choice than Christopher who is blood and he's been mentoring all along? Now here the writers could have taken the easy way out by making Christopher out to be a stunning pupil and protege by taking over the family like gangbusters and taking their crime sprees to new heights.
Instead, we thought it was a much better story point to go the way they did. Christopher was always a screw-up. It should have been no surprise to everyone that when given bigger opportunities Christopher was going to screw up in even bigger ways.
We thought it was always a wonderful plotline that Tony had great things planned for Christopher, but Christopher just wasn't cut out for the life. He lacked the skillset to become a great mafioso & thereby Tony had to resort to following behind Christopher cleaning up his messes because he had a great love for Christopher living in denial that the boy wasn't cut out for that kind of life till the very end when Tony finally realized that Christopher was endangering their entire mafia operation and very hard choices had to be made.
Sometimes the people you choose to follow in your footsteps don't have the motivations or abilities that you think they have or should have and the Tony/Christopher story arc showcased that point perfectly, in our IMHO.
But yeah, if you thought Christopher was going to shine when given his time in the son becoming the new Al Capone or something then yeah...you were sorely disappointed. However, if you go back and look at the early Christopher episodes it's easy to see he was in way over head and wasn't taking things anywhere near seriously enough.
Oh, one last unrelated thing...Rod Serling was magnificent. There's never been anyone remotely like him anywhere.