Re: We know all that!
I don't think that the viewers are analysing every aspect of the series, it is Dr Who after all and not a scientific journal. What all Science Fiction asks us, the reader or viewer to do is to 'Suspend our Disbelief' temporarilly. Some do it with great writing, be it in print (HG Wells) or as a great script or by 'Razzle Dazzling' us with unbelievable special effects (Avatar).
What a lot of 'Legacy' science Fiction, like Dr Who and Star trek have is a Fan Base that is very wide ranging in AGE. There are Who and Trek fans in their sixties and seventies. Many of them were so inspired by these programmes they carved out careers for themselves in academia and the sciences and are very sophisticated thinkers. But, they still have the ability to suspend their disbelief just as long as their intelligence isn't insulted. All that the film and programme makers have to do for Sci-Fi fans of all ages is just, as a minimum, make sure that the story they are trying to tell is plausable...thats all, just plausable.