I doubt that Peter Boyle (Frank) could've done another season. I once saw it said that his health started declining during the final season. Right me if I am wrong there. His passing was on December 12, 2006. The last episode was shown in May of the year before.
God bless you and his family always!!!
Holly
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Also, I disagree with the idea that it should have gone to season 10. I don't think they should have done season 9, although there were a few memorable episodes in that abbreviated season.
Another thing: the final episode was routine, other than the zoom out at the table, at the end. They could have maybe done another clip show.