the vast majority of rewrite movements I've observed or been part of are clearly, unambiguously due to the fact that:

- the legacy system is a spaghetti code nightmare - with arcane and manual deployment process - years of band-aid hacks layered over every component - inseparable two-way dependencies across all bounded contexts - inscrutable data structures - nearly impossible to debug - utterly fragile and mostly undocumented - riddled with dead code and unused components that merely confuse the poor soul tasked with debugging - often so intertwined with dependent systems that there's a dev "environment" shared across programmers because no one can develop on their machine.

THIS is why your programmers fantasize about a big rewrite. Every time.