UI/UX does matter if that is the problem you are trying to solve. If users can't use your product because they don't understand it, it isn't solving their problem.
Source: We sign up people because, and these are quotes from sales calls, "You have a good font size, easy to read", "I won't have to train new staff on how to use this correctly".
Of course, if your core value proposition is usability then obviously it matters. I think this is true for many of legacy Saas products. I was more talking in terms of b2c where there might not be a solution to a user's specific problem. (In your case, the solution is there but impossible to use so its similar to no solution available)
