Why wasn't Gomer promoted?

Maybe they should have just ignored the passage of time and never mention how long he'd been in the corps.

Nowadays they'd have him moving up the ranks and assuming greater responsibilities, yet somehow Carter would always remain his superior. Eventually they'd leave the Marines and go into business together. Thankfully, Jim Nabors knew when to quit.

It would have been fun to see a finale episode, though, where they wrap up the series. But where could they go other than to have Carter think he'd gotten rid of Pyle once and for all, only to have him return like a bad penny? They'd already done that in several episodes (a frequent quibble is that Pyle's drill instructor never would have remained as his platoon sergeant, but I think they explained it by having him put in for a transfer, never anticipating he'd get stuck with Pyle).