One reason I’ve seen for past efforts like this to fail is that COBOL uses fixed-point decimals instead of floating point. As a result, the decimal math functions are largely integer based and blazingly fast without losing precision.
When we translate COBOL to modern languages though, we end up either needing to use floats (which famously lose precision) or reimplement fixed point math in the new language, which often ends up significantly slower than the COBOL code. And when you’re streaming millions of financial transactions/sec you really need both.
I’m hopeful that somebody will crack that nut and free the finance sector from the 1970’s… But there’s more than just the usual challenges of a major rewrite.