Experts turn fools only when you start following your dreams. They have their own mental models, their own vision, way of thinking and knowledge which would never be a complete picture.
You may be visioning something else in that segment which no body might have thought of in that depth or in that manner.
It is difficult to go alone in this conflict but who said it would be easy for an entrepreneur anyway!
Look at these examples:
“There is no reason anyone would want a computer in their home.” – Ken Olson, president, chairman and founder of Digital Equipment Corp., 1977.
“While theoretically and technically television may be feasible, commercially and financially it is impossibility.” – Lee DeForest, inventor.
That said, listening to these perspectives does help to iron out nuances, gain knowledge and perspectives to fine tune your vision.