How far can we consider demand forecasting as the most important realistic feasibility indicator?

I can say that "demand forecasting" is the key start of any business.
Without having that forecast, no further steps can be taken towards the study of any new business.
Business is always made when there is a demand of something, so forecasting the demand is principally needed to assist the business decision.

Demand determines the sales forecast which ultimately affects the production forecast.

 

So, it's very important factor to assess while preparing a realistic feasibility study.

You can consider “Demand Forecasting “as an important indicator and not as the most.
The most important indicator in feasibilities studies should be the market structure studies to estimate your market share followed by state of art technical studies.
Regarding what drive markets, industries, or brands? I could say it could be by either of them, markets moves based on interpreted opportunities and threats.