How prevalent is `C/C++` in machine learning development?
As you already understand, the vast majority of the data science work is made with rather high level languages such as Python and R. So it's not a matter of prevalence, it's a matter of which part in the big world of data science you want/can do with your skills and your tools.

Imho inventing new models requires:


strong theoretical background in maths and statistics, in-depth knowledge of existing estimation/inference methods 
good understanding of computational complexity and (preferably) algorithmic optimization methods.


If additionally you implement your models yourself (that's not necessarily the case), that's where you probably need to know low level languages such as C/C++, because computational efficiency is crucial when people are going to use the model with massive datasets which require a lot of computation.