I think many of the UBI supporters know "UBI will replace all the existing welfare programs" is just a sales pitch, and not actually what will happen. For the reasons mentions in this article, UBI will end up being in addition to existing welfare programs, not instead of them. This deceit allows supporters to claim it won't be astronomically expensive, and then once it has become policy and it turns out people aren't using the money on necessities, they will say "oh, I guess we need to keep all the other programs too!" Thus achieving their actual goal of massively growing the welfare state.

UBI is fundamentally about ratcheting up redistribution, not ensuring that people's necessities are met. Programs for the latter will continue to be necessary.