https://www.metv.com/stories/ron-howard-on-the-andy-griffith-show-for-andy-the-show-was-never-the-same-after-don-left

Griffith was losing a friend and a partner in comedy.

Maybe TAGS should have transitioned to Mayberry RFD after Don left. Perhaps Andy could have remained in a slightly lesser role, if his ego would have allowed it.

I think Andy could have offered Don a lucrative deal to stay with the show. We'll never know how it all went down because both Andy and Don kept quiet over the matter.

Looking back it was foolish to allow Don to leave the show, but both Andy and the producers underestimated how valuable Don was to the show. His guest star episodes post Season 5 proved that.