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.

From the document itself: "Despite Go’s simplicity, there are many subtle cases in the optimizations and other rewrites performed by the compilers, and it would be foolish to throw away the 10 or so man-years of effort that have gone into them."

Yes, this is indeed one of the questions that the article addresses in the "Alternatives" section.