Suppose Alice makes $100,000 and Bob makes $20,000.
In a system that taxes at 5% up to $50,000 and 30% thereafter, the state gets a total of $18,500 from Alice and Bob.
In a system that taxes at 5% up to $50,000 and 40% thereafter, the state gets $23,500 from Alice and Bob.
In a system that taxes everyone uniformly at 25%, the state gets $30,000 from Alice and Bob. This is not a smaller number and 5/6ths of it is from Alice. In fact, you can use that money to give them each a $12,000 UBI, leaving Bob with an effective tax rate of -35%, delete all of the transfer programs that obsoletes and still have some left over for transit and uniforms.
In a system that taxes at 5% up to $50,000 and 30% thereafter, the state gets a total of $18,500 from Alice and Bob.
In a system that taxes at 5% up to $50,000 and 40% thereafter, the state gets $23,500 from Alice and Bob.
In a system that taxes everyone uniformly at 25%, the state gets $30,000 from Alice and Bob. This is not a smaller number and 5/6ths of it is from Alice. In fact, you can use that money to give them each a $12,000 UBI, leaving Bob with an effective tax rate of -35%, delete all of the transfer programs that obsoletes and still have some left over for transit and uniforms.