At last, The Armageddon Factor limps to the finish line, thus closing the Key to Time season. I've actually enjoyed this last story of the season, as well as the whole concept of the Key to Time season, in general. In fact, as this season was my first prolonged exposure to Doctor Who as a young 'un, I was under the general impression that The Doctor was always searching for something specifically, so the notion of a grand story arc such as this didn't seem as jarring to me as it may have to long term viewers at the time.
The lack of scale really hampers the end of this story, though, as well as the lack of scale to the final scene. Whereas The Doctor's first meeting with a Guardian occurred in a strange, mystical world that the White Guardian dragged him to, the climactic meeting with the Black Guardian occurs within the safety and familiarity of The Doctor's TARDIS. It's a shame that this scene couldn't have been set in some a place that offered some sort of danger, but it wasn't. And look at the scene - even though The Doctor is supposedly being threatened by one of the most powerful beings he has ever come into contact with, he is 100% in control. Zero threat, zero danger, zero drama.
And then the giant reset button gets hit when The Doctor scatters the segments, enabling Astra to return to life and rejoin Merak in romantic bliss. Does this then mean that all the other segments were returned to where they were originally found? Is there a necklace hanging around one of the stones in the Nine Travellers? Did Garron finally get back his lump of jethrik? Is there now a shrunken Calufrax creating a black hole somewhere in space?
Whatever the case, the reset was both inevitable, in hindsight, and unavoidable. If the balance of good or evil was reset, as the White Guardian intended, then there wouldn't be much of an entertaining program to watch anymore if there was no evil for The Doctor to stamp out. It all seems, as the last scene would indicate, a season-long explanation as to why The Doctor installs a Randomizer in the TARDIS, thus giving the series a raison d'etre for appearing willy nilly in various places. It also gives Graham Williams the satisfaction that he's justified a reason for The Doctor's travels, as the moral ambiguity of the apparent randomness of his exploits was always something that bugged the producer.
On the whole, The Key To Time season was rather entertaining, and nice way for me, personally, to relive some happy childhood memories.