The business case is that over time when people want to add more features to it (which if you think about it, having to mod the existing code to add new features is a sign of bad code too!), it costs more. The probably new developer is met with having to do major work to the code they didn't write and don't understand at a time when they're trying to also add a new feature to it; or they workaround it making the new code unweildy and just exacerbating the problem! It may seem a valueless operation now, but in the future it means having to abandon the investment entirely. You just hope that you made enough revenue from it to warrant the reinvestment. That said, I see more and more that a company's code reinvestment is driven by the change in trends for software development practices (finding the devs with skills in your code stack), so maybe the rewrite is now the expected evolution of a system.
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