Dalek Invasion of Earth was a really ambitious project for the early Doctor Who Team. The story also was the first occasion where a featured player in the series left.
Susan’s departure from the series and the universe wandering clan is a somewhat sentimental moment, but also shines a light on a piece of history about the characters of the Doctor and Susan. Show creators never intended for Susan to REALLY be the Doctor’s granddaughter and add that attribute to the character later.
The writers never got the feel quite right – and the feel of this departure is a profile in abandonment in some ways.
Let’s take this scene in context:
1) The earth has been dominated by the mecha-Nazi race of Daleks for who knows how long.
2) the planet will need to recover and rebuild BUT
3) the planet also needs to recover from a worldwide plague on the order of the Black Death.
Susan goes goo-goo eyes for David and the Doctor locks her out of the TARDIS. Seriously??? In that place?
Look for our more extended review of the entire Dalek Invasion of Earth story in the next could of days.