If the banks are doing something predatory then that behavior should be stopped. Not being able to define it and then seizing an arbitrary portion of the profits isn’t going to stop it in the future. Hell, since profit is after expense, they can increase CEO salaries to reduce it and be taxed less!
Plus if the “crime” here is fleecing depositors with interest rate spreads, they should make them pay that arbitrary percentage to each depositor based on their pro rata share over the coarse of the year.
Otherwise you’re taking some pensioners’ low savings account interest and giving it to away to someone else entirely.