
Very critical questions, indeed.
I think it is very difficult to predict the future rate scale as the industry is full of gray areas. Lack of research (let alone rigorous research) is one prime reason; the other is the porousness of transactions taking place on the internet.
Perhaps better internet technologies will help the essay writing industry to grow mature enabling the writers and the customers to make better informed choices. Thus, although it may not be possible to bring the industry back to it booming period (when native writers were valued more than the ESL or EFL writers), it is also rational to argue that the quality would still matter in the coming days. So, a mature market will probably have segments of different types of products written by writers from different backgrounds.
I am not taking side with anyone, but it seems that in more general areas of essay writing industry (such as literature, management, tourism, etc.) the native writers will face growing competition from the NN writers. However, when it comes to certain niches, they can still maintain their edge over the NN ones.
I'd like to point out a very important area that has not been touched upon in the post above - the case of growing trend of essay and research writing in many countries other than US and UK (ESL/EFL). Many well-renowned colleges/universities (both local and with international affiliation) based in Singapore, Malaysia, Hong Kong, and even in India and Pakistan now require of the student to complete certain essay writing tasks. Thus, it adds to the riches of the essay writing industry.