I do not believe he was dropped I think Piers Brosnan felt he had done enough and wanted to move on. He also said that he was getting past it age wise.

" In 2002, Brosnan appeared for his fourth time as Bond in Die Another Day. “ I'd like to do another, sure. Connery did six. Six would be a number, then never come back. ” (Brosnan, on continuing playing James Bond, during promotion for Die Another Day.) Shortly after the release of Die Another Day, the media began questioning whether or not Brosnan would reprise the role for a fifth and final time, in the later titled Casino Royale. Brosnan kept in mind that both aficionados and critics were unhappy with Roger Moore playing the role until he was 58, but he was receiving popular support from both critics and the franchise fanbase for a fifth installment. For this reason, he remained enthusiastic about reprising his role after his initial contract expired, despite earlier reservations about doing so. Throughout 2004, it was rumored that negotiations had broken down between Brosnan and the producers to make way for a new and younger actor. This was denied by MGM and EON Productions. In July 2004 Brosnan announced to Entertainment Weekly that he was quitting the role, stating "Bond is another lifetime, behind me"; this is thought by some to be a failed negotiating ploy. In October 2004, Brosnan had been quoted as saying he considered himself fired from the role. Although Brosnan had been rumoured frequently as still in the running to play 007, he had denied it several times, and in February 2005 he posted on his website that he was finished with the role. In spite of this, rumours continued to circulate that he was in negotiations with the producers right up until Daniel Craig was signed and announced on October 14, 2005." Source and further information: http://en.wikipedia.org/wiki/Pierce_Brosnan So it seems that the producers do not want to keep the actors to lang in the role. I think they could have been more flexible because Brosnan was a great James Bond. But I think also that he is certainly not at the end of his actor's career.

Brosnan was unfairly dropped by Wilson and Broccoli. He had said repeatedly he would return for a fifth. He had brought the series back from the abyss Timothy Dalton had left it in. But, the producers felt they wanted a darker, more brooding Bond and Pierce didn't fit their vision of the new direction they were taking Bond. I, for one, really wish he was back. I have hated the last two Bonds and squirm everytime I think we have to sit through two more Daniel Craig Bonds.