There is a large communication overhead to this. You have to start a new project, which could take months to develop. In the meantime you still need to maintain

There is a large communication overhead to this. You have to start a new project, which could take months to develop. In the meantime you still need to maintain the current compilers, fix bugs and make improvements but all that effort becomes wasted. People stop working on the old code base because your new code base is more fun so it suffers and eventually you end up with a not-yet working new project and a broken buggy old project. With a translator work doesn't have to stop on the current compiler code base while the new version is developed, all that effort is saved and bugs are automatically fixed in both places so you don't have to maintain and synchronise two bug trackers.