It was never a real marriage, as Charles never stopped his affair with a married woman (the disgusting Camilla). He never made the commitment necessary to make the marriage successful, and it is doubtful that he ever intended otherwise.
Where is the romantic in you. Despite the difficulties and disappointment that duty placed in their path an elderly couple finally managed to marry the true loves of their youth, and live happily ever after.
Charles is married to the woman he always loved, and who loved him, instead of a crazy manipulative .... He did his duty, now let him enjoy his final years.