What can cause a blockage in an otherwise decent main sewer pipe?
You have a camera view and a Plummer that says the pipes are good . In this case "shit happens" the drains can be fine for years then you do some cleaning and a hair ball runs into a feminine hygiene product and the drain clogs. Since the Plummer is not trying to get you to replace the cast I would not worry because of the case I have listed seamed to happen with rentals after cleaning and new tenants.