"Usury" is still a word in use but now, for legal purposes, it means "interest that a lender charges a borrower at a rate above the lawful ceiling on such charges" rather than just any interest at all.

it is impossible for there to be "financial theory", for the simple reason that money is a human construct, just like the tooth fairie , easter bunny and varios other coolaid cults, but with different characteristics and rules. And now, with the end of scarcity the feeble rationalisations of printed money are no longer,worth there salt.