Your first question ignores the crux of the matter: from scratch rewrites of software that is actually worth rewriting are always failures in the making. While

Your first question ignores the crux of the matter: from scratch rewrites of software that is actually worth rewriting are always failures in the making. While the rewrite is ongoing, the original must undergo continued maintenance and this (a) doubles the workload of the team and (b) presents an ever moving target for the rewrite. This means that the effort that goes into piecemeal improvement is the minimal effort possible that can succeed. Successful piecemeal rewrites will result in gradual improvement, so if you don't see that gradual improvement over time, something is very wrong with the project.