I can understand why they would claim that was the reason but really Bewitched was canceled due more to the low ratings. ABC replaced one Darrin with another and made Tabitha and Adam adults six years later in the timeline for the Tabitha series. Clearly they would have had little problem with switching to another actress to play Samantha. If Bewitched had higher ratings ABC would have done whatever the non-IP infringing equivalent to what Time Lord regeneration is for witches to squeeze out one more season.