He's something like 96, and has been working on replacing himself for at least 10 years, maybe longer. He announced his retirement a few months ago. It would be irresponsible to not plan for his succession; training your replacement is arguably the first responsibility of a leader, especially when your timeline is long-term.
Buffet still has 30% of the voting rights which makes him pretty hard to replace in the theoretical situation where someone would have wanted to. He's retiring at the end of the year and remaining chairman of the board.
Publicly saying "we paid lots of tax" would be career suicide for a tech CEO.