> There's no way a VAT or flat tax would be progressive enough to fund the government without crippling vast portions of the taxpaying base.

That's what the UBI is for. You can have something like a 35% VAT with a $15K/year UBI and then the effective rate on someone at $60K/year is 10%, but at $100K/year it's 20% and it only goes up from there. This is very much in line with the existing effective rates, and you can make them whatever you want by adjusting the tax rate and the amount of the UBI.

It also replaces a huge swath of social spending programs because you're then giving someone who makes $20,000 a year $8000 more than they pay in taxes.