> 10% hurts the same no matter your income.

No, 10% of income doesn't hurt the same no matter your income. (Even if you ignore the relationship loose correlations between income and savings that can be used to cushion the effects of unexpected expenses, and assume neither party has any such resources.)

While fungibility pushes slows the decline compared to less-fungible goods, declining marginal utility applies to income, too, which means not only does a flat fine have less impact on the rich, so does a flat percentage.

(This gets even more true when you do consider savings, etc.)

Agreed, but a percentage fine is still a much more effective deterrent than a flat fee which becomes insignificant much more quickly.

Even better might be a percentage of disposable income but even that is not going to be enough and with more complexity in the rules comes more opportunity for creative accounting which again benefits those more well off.

10% of the worth of all assets then.

Id rather have it scale badly than not at all though.