Square is built on relatively modern (eg this century) tech stack. Modern tech stack are inherently more fragile than the real legacy stuff. The Mainframe tech that powers visanet has enormous practical limitations. But holy crap is it stable. Something about the dozens of abstraction layers that hadn't been invented yet when the original ISO payment standards and processes were first invented. The very close to metal and low surface area for breakage. And the benefit of decades of experience in these legacy stacks to shake out bugs. All of Visa's global commerce literally runs through just one of seven (mostly one of three) physical computers. Crazy stuff. But probably not sustainable many more decades. Eventually core networks will have to migrate to modern hardware, OS and SW stacks. That migration could create a lot of benefits, but I would not expect that maintaining the currently observed ~5-7 nines reliability to be one them.