Yeah - code rewrites are generally a thing to be avoided since the requirements of older production code is typically not fully documented, even if it originally had been. Production code accumulates bug fixes, performance tweaks, special cases, additional requirements, etc, with documentation and test suites not always being updated in sync.

Your rewritten codebase will presumably look much cleaner and more maintainable that what it's aiming to replace, but it's easy to miss the purpose/necessity of all of the old "cruft" which need not be just poor coding but rather later enhancements not anticipated by the original design whose implementation was therefore crufty.

Does anyone here work at Cisco, and have any insight as to why a rewrite is considered necessary and might be expected to succeed?

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It says "legacy" code.

Legacy code is typically a euphemism for old unmaintained code, no one knows how it works so it can be edited, and it probably doesn't align with current work practices.

Not to say a rewrite is necessary right, but avoiding rewrites of working code is how we ended up with more COBOL code than COBOL programmers in 2026.