None of these proposals are enforceable in any meaningful way.

Sure they are. They affect the actions of many companies that today think what they're doing is okay (or at least not illegal). Don't underestimate the value of substantially reducing a harm, even if it isn't eliminated entirely. And don't underestimate the value of making it easier to address the remainder by ensuring it's 100% illegitimate.

Regulate it today, and tomorrow, corporate legal departments will be very carefully training their employees to understand that it's illegal and they should never do it.

Currently, some countries have laws saying that you're not allowed to pay bribes, including foreign bribes. Consider how widespread that practice was when it was outlawed. Imagine if, instead of regulating it, those countries had said "oh, that's not enforceable and too many people are already doing it and it would affect existing business practices...". Instead, today, corporate legal departments will ensure that employees are trained to know they can never do that and they should report any attempts to solicit bribes.