And if Assignment Earth aired in Britain say 1970, if it was a success there, then we could have seen a second season of UFO and maybe the Doctor Who Spind Off Unit.
Especially since, if Doctor Who follows the same path as OTL, then 1970 is the beginning of the 3rd Doctor's run, which also took a "James Bond with Aliens" approach. (He was stranded on Earth for most of it.)
As the OP pointed out Assignment Earth was being toyed with in case Star Trek got cancelled.
Star Trek almost was cancelled after season 2 and the executives were reluctant to commision a 3rd season. When they finally relented they moved it to a different schedule to reduce ratings.
Once executives decide to kill off a TV show they eventually win.
IMO you can't have an Assignment Earth as well as a Star Trek season 3. It's one or the other.
I like the Assignment Earth episode and Robert Lansing's screen presence seemed to eclipse Shatner and Nimoy quite easily.
OTOH I think AE would have worked better as a movie than a TV show. A TV series would look like a Doctor Who/Mission Impossible/Man from Uncle fusion and they would run out of credible ideas very quickly before shark jumping.