> Once you have the foundations in place for account balances and the ledger, processing a payment isn’t that daunting. Those foundations, however, took a lot to build and evolve.

Pretty much. I've worked at places with PHP payment processing that worked just fine, and at a place with C++ payment processing (and no testers) and it worked just fine. I wasn't around when the systems were first built though so not sure if there were tears along the way.