As you say, loans with interest rates that high no longer exist. But you are still incorrect with how mortgages work in the US. You absolutely cannot suddenly borrow more with lower interest rates. Once again, the amount you can borrow is based off of credit scores and income, NOT interest rates. Sure, maybe you could borrow like $25k more, maybe even $50k. But that is nothing compared to your full loan, and no bank is going to see that interest rates are low and be willing to suddenly take on more risk with little gain. What would the bank's motivation be? They gain nothing but risk by doing that, so they don't do it. They do allow for more risk when interest rates are high, because they make more money off of loans. It's the exact opposite of what you are trying to claim.
The US is exceptional here, maybe not in a good way, however, we definitely are. I don't understand why you are trying to tell me that my lived experience is wrong and that your vague and incorrect assumptions about the US mortgage market are correct.