I don't think it's that easy to just predict the next 10-20 years of web design. The information technology industry overall is capable of changing it's direction overnight, and has done so on numerous occasions.

It actually doesn't worry me one bit, I've been able to benefit from the commercial themes offerings on numerous occasions; as a small firm (my wife and I) we are only capable of taking on so many projects at any given time. The ability to spin out a solution using premium themes and other similar resources has quite often allowed us to take on additional projects (extra income) where we couldn't have before. That has been a huge benefit to our growth.

The problem with premium themes is that for every theme out there, there could be anywhere from 1 to 1,000+ sites already using it. People who want to get online quick are loving that. People who want to take the online side of their business a little more seriously don't want that. There will always be a market demand for custom design and development so long as there is a world wide web.