you know im pretty sure the Doctor is going to meet new people, some of them could well be female (or the female of the alien species)yes after watching End of Time, I just dont see what harm will come to a show if it has a male companion. I just dont buy into this notion it HAS to be a female companion. Heck Clyde from SJA was a great companion in Whatever Happened to Sarah Jane.
Ratings. Take away the customary cute female companion and you might suffer a bit with straight males.
I see no storyline reason why the Doctor cant have a male companion as the main companion, and maybe a female companion who comes along for a few episodes (like Captain Jack)
Personally I would continue to watch the show, if it switched form one regular female companion, to a series which featured a new female hotty in each episode.