Especially for an episode that didn’t really have the Doctor in it. In Hartnell’s day, he would take a week or two off here and there and nobody would really notice. None of his successors managed to get away with it to the same extent though.
By truly bizarre coincidence, I banged in my ancient videocassette of Frontier In Space this afternoon, and in the first episode there is a 26th century newsreader played by Louis Mahoney. And there he is again this evening, playing the older version of Billy. (It would be a truly brilliant example of synchronicity if he had appeared in a Doctor Who episode first broadcast in 1969, but it was 1973. And, I see on further research, also 1975.)
Edited to add:
I say it again. That was one of the best episodes ever.