> You can't retroactively change the rules.

Why not? You totally can. It's not some fundamental law of nature that regulation can't be changed after some time. The loans were made under the assumption that they are not dischargeable, at that time someone could attempt to estimate what's the probability that the laws would be repealed or modified in some time horizon. That probability is going to be > 0. Is that risk large enough to change the business case for making the loan in the first place?

You could, but, in the U.S., ex post facto law is unconstitutional. Lenders have correctly gauged the odds of this provision of the U.S. constitution changing to be near zero.

https://www.usconstitution.net/xconst_A1Sec9.html

This would be an ex post facto change. The government wouldn't be retroactively making the lenders guilty, it'd be a change in regulations. If this change was unconstitutional then you'd have situations like a company with a contract to dump industrial waste into a river going "oh sorry, I made this contract based on the regulations back then, so it's unconstitutional for you to make me stop polluting"

If you had a contract that was contingent on your ability to dump your waste in the river, and then a law was passed forbidding the dumping of waste, you would of course have to stop polluting, but your contract would be null and void as well.

Non-bankruptable student loans aren't "regulations" they are specific contracts between students, lenders, and the US government.

You can't get blood from a stone either and that appears to be the only alternative the lenders are seeking. The many of these loans are bad, they were for more money than the education provided in income. If they don't become dischargeable or just get written off the system is going to collapse

If the student defaults, the lender still gets paid. The government steps in and pays them. The debt is then essentially owed to the government rather than the lender.