The point is that if you convert away from COBOL to a more modern language, you can also move away from Z-series hardware to commodity x86 and ARM servers. That's why this announcement affected IBM's share price.

IEEE 754-2008 defines decimal floating point arithmetic that is compatible with COBOL and is usually implemented using the Intel Decimal Floating Point Math Library on commodity hardware.

For a typical core banking ledger application, the performance cost of a software implementation of DFP (vs. having DFP hardware instructions) is pretty low, and greatly outweighed by the benefits of being able to use commodity hardware and more maintainable languages.