Seen this before. And this, my friends, is why actually the Lannisters don't pay their debts. And why the Tyrell growth is an export-based (wine, weapons and grain) economy, that guess what, has many of those three in great demand fucking everywhere. Highgarden castle itself wouldn't have developed much, but the Rosewood (a massive, massive forest, based of non-coniferous and coniferous trees, east of Highgarden), and particularly Hightower/The Arbor would look almost like merchant republican cities, really, really flowing with trade revenue. The Tyrells didn't ever do loans, if they did (as we see Mace doing in the tail-end of S5), they're through the Iron Throne. It's been a century or two and a new set of rulers who are probably paying it back prudently in the absence of any war whatsoever, so no doubt, that has worked out well. Even the Crakehalls shouldn't have had to borrow money- in fact, actually, they could have borrowed money to the Riverlands, to restore their completely destroyed economy in the wake of the War of Five Kings...though again, could have easily been paid back.
The real question then is, what does Braavos look like? Remember, they were rich before. Now that....is some scary, scary fucking power there, considering it is likely the debt has been paid following the Targaryen restoration- new rulers, same loan, but perhaps a little more sympathetic to allow the Targaryens to begin paying back. Money can buy everything, mercenaries, loyalty, kingdoms, anything at all, for anyone who would like it.