I note the comment that "When rebellion became less fashionable, so too did Doctor Who." I've heard theories that the bleakness of Blake's 7's season 4 and the final scene were ordered around the same time so that a group of rebels who liked blowing things up would not be seen as either admirable or successful.
I've not seen that much Dr Who (I barely remember what I saw as a kid, and since then have only seen the first couple of series on reruns two years ago) but I'm looking forward to the new series.