DuracellEnergizer said:

Personally, I find the idea of making the series finale of one show the pilot episode of another entirely disagreeable - especially considering it was a pilot that was never picked up.

That wasn't the finale of TOS, it was the end of the second season. Mr. Seven, Isis, and Ms. Lincoln went on to have their "interesting experiences" in a series of novels published a few years back. Gary Seven has appeared in the comics over the years as well.

Backdoor pilots were stock and trade in the 60's. The Andy Griffith Show got it's start as an episode of The Danny Thomas Show, which then begat Gomer Pyle and Mayberry RFD.