That assumption doesn't necessarily hold for a Doctor companion though. He could arrive on an alien planet where the aliens are around our level of technology (and coincidentally speak english!) and it could still work. It could be a little more interesting as the alien could be asking questions about earthlings...
Could be even more interesting than the human companions, who we're meant to believe are both simultaneously brilliant, able to save the universe and solve complex logic problems, but also ask a stream of mundane questions (largely so that unfamiliar concepts can be explained to a varied audience). At least with an alien visitor there would be a reason for asking questions about everyday things. They'd have to be mostly human in appearance though, as the majority of the storylines are centred around human characters (I assume for budgetary reasons) and they'd need to blend in more often than not.
The cost of feathers has risen, even down is up!