Anybody can write a Parker story. With John Gardner and the James Bond books, since you mentioned him, the pleorbm was not the grasp of the character, and Gardners style didnt need to match Ian Flemings. Ian Fleming had no definitive style of writing (as in, putting the words together) but its the opposite with Westlake. Richard Stark had such a distinctive style of how he assembled the words that part of the fun of the Parker books was just reading those words even when the books were repeats of what had come before. No other author can capture that style. For many years, as you know, Westlake could not write in that style, so he didnt produce any Stark books because why simply imitate?