Every single proposal of UBI has included it replacing all other welfare benefits

Only so that their cost calculations don't go crazy and seem sustainable. No one in the UBI side is even remotely interested in deleting previous welfare

Now you're just making stuff up to sustain your apparent hatred of everyone "in the UBI side".

I think you're working from some wrong assumption who these UBI proponents are, and what their motivation is. Just to give you an idea, Wikipedia lists Elon Musk, Tim Berners-Lee, Mark Zuckerberg, Sam Altman, Bill Gross, and Tim Draper, among others. These aren't lazy know-nothings trying to get you to finance their lazy Tuesdays.

I'm more concerned about people in governments, not rich millionaires. The only way it could work would be to create a UBI of 0 at the beginning and publicly say to increase it only every time a social welfare is deleted. I have huge doubts about the doability of this solution.

I don't have any issue with billionaires believing into UBI and sending cash to Kenyans. I don't know their true agenda, but they might gain some political influence / power when they'll try to implement this into western countries (and it will be with our money this time of course, not theirs).