It is well known that big software rewrites are often failures. They fail because they always look easier than they really turn out to be. To replace a system that has been around for years is likely to take years to rewrite. Plus you have to add in all that time required to fix new bugs, add the features that were missed, and handle a whole new set of discovered edge cases. While the rewrite is happening, the existing product is not getting the attention it needs. Bug fixing and enhancements stop and customers become frustrated. This can give the competition an advantage. They may gain market share by delivering features customers want and need while you spend two plus years rewriting and stabilizing your product.
If you haven’t already read it, check out “Things You Should Never Do, Part I” by Joel Spolsky from over 10 years ago that is still very relevant.