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 :)
Having a competitor is a good sign, but how do you know they're profitable? How do you know they're solving the problem well enough that they're even worth imitating? How do you know they're not frustrating the hell out of their users? If you speak to potential customers, you'll know.
Not saying that you need to have full personas, in which you include ALL motivations, demographics, customer journeys - the works - but it helps to have something on paper that you can refer to.
The beauty of it is you can always change them!
No matter how certain you are talk with your customers!
I talk to people, but I need to talk to more.
Don't assume anything, get real leads and/or sales and talk to them about your planned offering and pricing.
Use data.