Most big banks run on COBOL, new Fintechs run on micro-services powered by Go, JS or whatever's trendy at the moment. I trust software with a proven track record of several decades with no considerable downtime more than architecture that's currently considered to be "best practice".

NB: Don't object any of the new languages, I've never written code in COBOL or for a mainframe. But I've also never developed software with that kind of requirements.