After reading The Are The Voyages I found myself revisiting sections of another "making of" book regarding Mission: Impossible. There are a lot of parallels between Mission: Impossible and Star Trek.
I wonder what difference it would have made to Star Trek if Mission: Impossible had not sold at the same time. Perhaps if Star Trek were the only "risky" venture for Desilu in 1966, the studio heads would have supported it better.
I liken these two series to twins... one gets the better grades (think ratings) and thus gets cut a little more slack from mom and dad... and sometimes the other twin is unfairly overshadowed.
One thing's for certain... if anyone had suggested in 1969 that Star Trek (heading for cancellation after a poor third season) would spawn a dozen movies and several spin-off series, and be considered much more of success than M:I (which in '69 was still getting high ratings and had three seasons left to run), they would have been committed!