100% this. I've also seen a lot of cases where the rewrite is partial, and integrated with the legacy system to preserve end to end functionality, as they don't

100% this. I've also seen a lot of cases where the rewrite is partial, and integrated with the legacy system to preserve end to end functionality, as they don't have enough time to do the full feature set, migrate all the data from the legacy system etc. You then end up with a hydra system, where you need to understand both legacy and rewrite systems, and their interactive, increasing the complexity even more!