Thanks for all of your opinions. Thanks to you, I've had more views for that sentence.
***
By the way, in the beginning of this episode, when John said goodbye to the newsvendor, I heard "Let on, walk" (from the cab driver?). I still have no idea for a sentence "Let on". Can you explain this sentence to me? Thanks in advance!!

Harriet wrote:

I'm not a native speaker either, but I will try

SH: His body was never recovered.
MH: To be expected when one pushes a maths professor over a waterfall. Pure reason toppled by sheer melodrama: your life in a nutshell.

The sentence is about the end of Moriarty, a maths professor ("pure reason") who had an end unworthy to his abilities - being pushed down a waterfall ("sheer melodrama"), and includes a warning to Sherlock not to end like that, get himself in emotionally overloaded situations he can't cope with and thus waste his potential. "Your life in a nutshell" also states that Sherlock is on the best way towards it already (in Mycrofts point of view).

Oh, I just understand it now.
Thank you