There is the broader point that ratings matter much less when Dr Who generates income via international broadcast rights, the Doctor Who Experience, global tours and events, the licensing of novels, toys, games, exhibitions and so forth. However, what lower ratings might be more likely to encourage is seasons of specials, seasons split over 2 years, and so forth. They need to keep making enough Dr Who to keep interest in all the other stuff, but unfortunately we might have to face that this might mean that we lose a series or half a series every 3-4 years.
Of course, the rugby is an important point - I expect that the BBC will be bracing themselves for a ratings battering this Autumn. It's not just the live match time - it's the people who've gone to the pub or a friend's to watch the game, the people who switch over to the secondary match on ITV4, the people who delayed cooking so they could watch the game live - the matches will disrupt ratings all night.