That would've worked. Maybe they move to another city, to explain why we don't see SJS, and they have some gizmos of their own that Mr. Smith made for them, or that maybe the Doctor left them in an unrecorded visit. But you gotta have K9 -- say SJS asked him to go with them to keep them safe. Another possibility might've been to have another former companion take over as their mentor. The Martha Jones Adventures, anyone?