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.