There are many "common practices" taught in IT that I disagree with. One of them being that "haha, silly to think that rewrite would be faster". Especially when the alternative is to have Frankenstein's monster of frameworks and eras.
Sure you can think that you stitched those N generations of tech together perfectly and user won't be impacted and you will be able to peacefully develop those projects side by side.
They will. You won't. It will be pain. For everyone. Do the rewrite!