Why end the show? Shortly before she passed away she had an adventure with Jo Grant. Why not have Luke and K-9 pay Jo a visit and give her a letter from Sarah asking her to protect Luke's secret and come to live at 10 Bannerman Road and continue the fight and protect the Earth. As the show ends, a new title screen appears formally announcing The Jo Grant Chronicles. This would be an excellent epitaph. An organization started by Sarah and handed down to the next generation.