Comment by rurp

It's been some years since I watched the show so I've probably forgotten a fair amount, but I remember it differently. I recall Tennant arguing at various points that it's probably the more obvious suspect/explanation. It's the whole you're probably hearing horses not zebras thing. Which in reality is the more competent approach. Crimes usually are committed by the most obvious suspect and pursuing more obscure theories is a worse approach.

But of course in a TV universe that's completely flipped on its head, nobody makes shows about normal straightforward cases.

This same conflict bugged me about the movie Zero Dark Thirty. The main analyst is 1000% sure that her hunch is correct and is constantly aggressively adamant about it, despite a lack of hard evidence. The others analysts are shown being much more rational, giving probabilities to their assessments and grounding conclusions in evidence. But since it's a movie of course you know the heroine is going to be correct and all of the other people seems like indecisive fools. But in reality someone who acted like her would be an absolute train wreck and the sober rational ones would be getting things done consistently with far fewer screw ups.