Most western social democracies already have an overlapping network of benefit systems that are essentially a patchwork UBI in disguise. The system is designed not to go without food and shelter, but the exact mechanism varies depending on what population group you belong to. There's assistance for the temporary unemployed, for students, for people with disabilities, people with long-term illnesses, etc. There's pensions for those that have aged out of the workforce, there's assistance for children. And this is only scratching the surface. Even working people get a fixed basic tax deduction under most taxation systems.

I'm not convinced that the sum total of all these benefits plus the current infrastructure that is needed to maintain, dispense and detect/punish abuse on them is significantly larger than the total expenditure on UBI.

Of course, all the people that you let go from the benefits departments initially will end up on UBI, so that's not really a big win. But eventually they will find employment elsewhere, hopefully.