Here is what the Lord said to Peter when he was caught fishing rather than being about his Father's business:
Jhn 21:21 "Peter seeing him saith to Jesus, Lord, and what [shall] this man [do]? Jesus saith unto him, If I will that he tarry till I come, what [is that] to thee? follow thou me."
He told Peter that it was none of his business what the Lord had in store for John. His business was to follow Christ. After his hypocrisy in Galatia and his brief statement at the Jerusalem Council we don't hear anything about Peter except as an illustration in the first letter to Corinth and his two letters.
Prophecy is not of private interpretation, that is why the Holy Spirit came and leads each believer into all truth. It's not what I think but what the scripture says. "Believe on the Lord Jesus Christ and you will be saved."
Peter was not "caught fishing". St. Peter was the first to recognize the resurrected Christ and the first to jump in the water to greet Him. Must you insult the Lords' chosen apostle? The conversation about St. John's death and the Lord's coming directly follows the Commission of Peter and does not contradict that.
Christ knew that St. Peter will follow Him. And Peter did. To believe otherwise is to either disbelieve the Gospel, or disbelieve the commission of Peter given by Christ and described in the Gospel.
This is so good I added it to my Protestant Gospel collection. It is on my profile page at the end.