How can theoretical modelling be converted into viable product?
I'm assuming that you're really asking, "how do I do something useful with modelling and theory."

The easiest answer is to work in a modelling and simulation field that makes useful products. Computational Electromagnetics is used a lot in RF, Finite Element Analysis is used in mechanical product design.

The broken part of your reasoning is that "more expertise = more practically useful." This is wrong. Usefulness is an instrinsic property of a task. A task is either useful or not useful. Expertise is only important in as much it allows you to perform a task. Therefore, you need to:


Pick a useful task
Have enough expertise to perform that task.


Sounds obvious, but perhaps it isn't.