Not to mention that David Jason was more likely to pull in a wider audience of non-Discworld fans than an unknown (or lesser) actor, even if that actor fitted our concept of what Rincwind should look and behave like. Unfortunately WWD is right. When it comes to film and TV, money and ratings count far more than accuracy and fan's opinions (in whatever genre, the book is nearly always better).
The difference between fiction and reality? Fiction has to make sense.