It is a bittersweet little tune.
I think you're right about Frank's departure. Joel leaving was big, but once Mike settled in (and he did admirably quickly) it seemed this could go on twice as long (and it did.) When Frank left, it felt more like "you can't have this ensemble forever." That said, Season 7 (right after Frank left) is my absolute favorite season. It benefits from being the shortest season, but there's not a bad episode in there. All of the movies are their own kind of dumb/weird, the riffing is solid throughout, and the host segments were some of the most experimental of the Mike era.