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.
The problem is that these companies do receive subsidies during lean times, in both energy and banking fields, whether you want it or not. They are too important to be let to fail, and the governments will either subsidize them, bail them out or just plain nationalize them. In the case of Greece banking crisis, they nationalized the losses of foreign banks...
So a windfall tax makes sense in these industries, maybe in others too.
Utilities are similarly bailed out, because what are you going to do, leave people without water and electricity?
If we have risk floors, we should also have profit ceilings.
Be surprised, then. European regulators typically influence consumer prices, one way or the other, but profits are not capped.
[1]: https://wgme.com/news/local/are-private-or-public-electric-u...