Umm, loans were already commonplace in 1400s. There were banking families like Fuggers and Medici and so on who basically financed Spanish Empire and Holy Roman Emperors and so on. HRE Emperor Ferdinand II was millions of ducats in debt for Bavarian duke Maximillian and many other nobles and towns. Loans and subsidies might have been the one most important way how kings financed their wars before efficient taxation, as taxing needed always consent of the Estates. (which were so important they should be main game feature, btw)
I personally like how elegantly simple the loan system is right now. I haven't really abused it, though. I guess there might be balance issues, especially if you stack interest per annum.