When older software does not completely meet requirements anymore, but still functions good enough to not alarm business, complaints from people working with the outdated software might be dismissed.
This can cause people to think of their own ways to compensate for the perceived lack of software capabilities. A landscape of unofficial tools and processes can grow, comprised of Excel sheets, macros, tiny undocumented tools written by well-meaning hobbyist developers, and unsafe usage of the outdated system.
This poses a business risk, as such unofficial tools are often not well documented or tested, not under version control, or only usable by their creator. If, for example, an Excel sheet that compensates for a financial system that cannot cope with new regulations gets lost or contains bugs, the company might suffer financially.