Where do you live that politicians aren't constantly trying to remove welfare benefits?

This is why UBI will never become law. Once you start talking about taking away benefits (Social Security, Medicare, SNAP, Food Stamps, HUD Housing Subsidies), people will freak out. The backlash will be very loud.

Look at the tax reform debate now: When they proposed taking away deductibility of mortgage interest or state and local taxes deductions, I started seeing headlines from the New York Times and Washington Post about how this a “Middle Class Tax Hike”...

I would compare UBI to the "second system syndrome" with software developers: If only we can start over with an entirely new platform rather than addressing the technical debt in the current system, all would be well.

Look up the Welfare Reform Act of 1996. And with UBI you'd be replacing benefits, not removing them.

France. How are politicians incentivized to reduce welfare ?

Voters and donors, same as everything else. Even if there's no voter resistance to increased spending, implementing UBI without raising taxes or decreasing welfare would destroy the economy before your next election, so there's a clear incentive to not do it.

Every single proposal of UBI has included it replacing all other welfare benefits. That simplification, and the resulting reduction in administrative overhead, is among the top arguments for UBI. Also: spaces before question marks look strange.

  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).