You know, part of me is kind of glad they let Liz go rather than dumbing her down (which is almost certainly what Barry Letts would have done if he'd decided to keep her).
Just look at the Brigadier. In The Web Of Fear
he barely flinches when the Doctor tells him all about the TARDIS. In The Invasion
, it's implied he had a personal hand in the formation of UNIT, specifically because like a certain agent on The X-Files
, Lethbridge-Stewart Wants To Believe. Spearhead From Space
basically draws this even further, painting the Brigadier as the believer and Liz Shaw as the Scully-esque skeptic (years BEFORE Scully, I know I know).
Bascially, before Letts got his talons into the series, the Brig was a competent and professional soldier... but he was also a whole lot more
than that. He was also an intelligent man who, when presented with rational evidence, could be persuaded to believe anything.
(produced by Letts) punctured that, by forcing Lethbridge-Stewart to choose between his duties and his beliefs. He pressed the nuke, and the Doctor hated on him for it.
But this is Barry Letts' producership for you. Over the next five years, the UNIT crew became imbeciles.
The Brigadier became a dumbass whose primary role was muscle, nothing else, culminating in some excrutiating Idiot Ball moments in The Three Doctors
where he can't believe he's landed on an alien planet. Sgt Benton gradually turned into comic relief, going from the Brig's right-hand-man to being that guy who wears a nappy for the Everybody Laughs ending of The Time Monster
. And Jo Grant was Barry's antidote for smart intelligent Liz Shaw: Jo is a dumb broad, whose primary role is to make the tea and ask the Doctor lots of probing questions. Oh, and she got into UNIT via nepotism rather than on merit (her uncle in the U.N. arranged for her to be assigned).
Seriously, these guys are Earth's first line of defense?