Would getting the Lennon letter promptly have saved this hack's career?

Danny's career didn't need saving. It was apparent that he was quite successful. Remember the billboard advertising his Hits Volume 3? Danny thinks that if he got the letter and called John, that John could have saved him from the trappings of fame - drugs, ego-mania, etc. -- and ultimately ignoring his only son. Hence, the pilgrimage to NJ to right a wrong.