I think that a real bubble requires margin. It's not just that people are buying because the price is going up, it's that people are buying with borrowed money because the price is going up.

That ends badly. It ends badly for the lenders. So when it starts to look like that's what's happening, a perfectly reasonable response is to change the margin requirements. When the circumstances are normal, use the normal margin requirements. But when the circumstances are abnormal, of course they should adjust.