BI means that BI is the major form of monetary welfare assistance, but it doesn't mean it's the only safety net. Providing emergency shelter and food assistance for those who might otherwise starve or be homeless is actually pretty nearly dirt cheap, and there's no good reason to simply discontinue doing things like that (both privately and publicly run) even with BI.

Also, who cares about fraud? You can't solve the free rider problem so don't worry about it. It's an equivalent problem to letting the guilty go away because we're so concerned about avoiding incarcerated people or protecting people's rights. Yes, murderers absolutely do go free sometimes because we have a robust (sometimes) system of civil rights and legal protections, but that doesn't make the system worthless. Similarly, there will always be free riders who take advantage of any system of welfare more than they should. But realistically those people are not a serious drain on the resources of a rich, industrialized nation, and the benefit to the public at large from providing robust welfare support is more than enough to offset that problem.