Marginal rates are weird because you can compare your marginal rate against every decision, but at that point, you aren't comparing every dollar earned to every dollar spend, so effective rates seem to be more useful (marginal would be useful if I was considering working more, I think). Anyways, as someone with a high effective tax rate, I'm totally fine only keeping 60% (or keeping 40% if we added another 20), if in return we all got UBI.

Again, a UBI will in many cases reduce marginal rates, because no more means testing. Right now if you want to hire a maid to clean your house, you have to overcome the loss of government benefits to them from taking on more work hours.

Meanwhile at the top the difference would be less than an increase from 40% to 60% because the UBI is replacing all of these other programs which would no longer have to be funded, or could be made strict alternatives to the UBI which would make the UBI less expensive.

People whose marginal tax rates would be reduced by the implementation of a UBI are probably not hiring maids either before or after the implementation of UBI.

It's not the employer, it's the maid.

To get the maid to show up, you have to cause their bank balance to increase by at least $10, after taxes. If the government is taking 80% of what you pay them away in food, housing and education assistance, you have to pay $50 to get them their $10. Get their real marginal rate down to 40% and you can pay $20 while they enjoy a surplus of $2.