My $0.02.
If you feel like the problem exists, start building. But, if you think the problem is solved, then it makes no sense to proceed with it.
For a startup, people use your product because they have a problem to solve. It does not matter how bad your UI/UX is, how drastically bad your flows are, if you are offering a solution no one else is, you will have users. It just needs to work. The more pertinent the problem, more the users will be annoyed if it does not work. Ideally, your idea should be one of the solutions to the problem, and the entire initial phase is there to establish two things: 1/ Whether your solution works? Can you find a unique solution for the problem? 2/ Do a lot of people really have that problem?
For example: If you want to get into ridesharing and taxi market today, it will be difficult. But there is an open case where the wait times and variability annoys a lot of people. That is, if I am taking an Uber, I dont know how long will I need to wait. The app says 5 mins, but can be anywhere between 5-15 depending on the match. Sometimes, I have waited for >10 min and the driver cancelled and had to wait another 15. If some app can tell me exact time - or even let me schedule a ride - I would be happy to pay extra for that given it saves me the time and frustration. Not sure how many will have the problem, or it is good enough for others, but I will prefer a cab which allows me exactly this - even at a higher price point.