I believe the appeal to be (though fallacious) that with a ground up rewrite you already have an entire functional specification of the system you need to build

I believe the appeal to be (though fallacious) that with a ground up rewrite you already have an entire functional specification of the system you need to build. No need for annoying customer engagements, requirements analysis, spec-ing, scoping, etc. With these rewrites do you just abandon maintaining the legacy system and focus on the new one? I find it immensely more satisfying to "rewrite" a system using the Ship of Theseus approach (aka Strangler Pattern). If there are no tests, the first (and exciting) challenge is to put some behavioral tests in place in an attempt to protect it from the changes you are about to make.