First understand what their answer means. Not wanting to manage the company may be different from not wanting any involvement as owners. Separate employment, management and ownership before assuming that family continuity is impossible.
The company needs an operational succession plan whether or not family members take charge. Discuss what the family wants to preserve: the brand, business activity, assets or family ownership. External management, successor development, ownership reorganisation and a sale deserve assessment in context. A conversation can begin without committing to any one outcome.