I dunno... to me, it seems pretty par for the course for British TV to give each season its own distinct storyline, resolve it, then start something new next season. And by the same token, annual cast changes have a fair amount of precedent as well. Look at Doctor Who
. Even in the original series, it was unusual for a given combination of Doctor and companion(s) to last more than a year. And in the new series, there's been at least one change in the regular cast every season like clockwork. (To a lesser extent in the upcoming season, since the only change is Arthur Darvill going from recurring cast member to main-title regular.) Same with Torchwood
. The Sarah Jane Adventures
has kept basically the same regular cast for the past three of its four seasons, but only if you don't count K-9.