[corford]

Assumptions are inherently risky but ultimately you always end up having to go with your gut. That doesn't mean you shouldn't do the background research and talk to people first though! Even if you are right, you'll very likely discover that the proto idea you have now needs changing or tweaking in ways you would never have imagined had you not talked to some people in your target audience first.

I'm actually working on something right now that started like that. There are a load of large, well established incumbents operating in the market I'm about to try and pitch a tent in but none of them offer a service that does what I want. My gut told me I wasn't the only one who'd prefer my approach to the problem, so I talked to a bunch of people, refined the idea a little, did a load of research and when that all came back positive I quit my job and started building it. I'll let you know how it pans out :)