This isn't a "rewrite the code from scratch", but instead is an incremental but aggressive refactoring. There's at least one wildly successful example of this s

This isn't a "rewrite the code from scratch", but instead is an incremental but aggressive refactoring. There's at least one wildly successful example of this successfully being done for GCC, which is a project even bigger and more complex than Vim. Around GCC 2.8, it was forked into EGCS where major changes were made. This fork was so successful that it then became the official branch of GCC, and modern versions of GCC are direct descendants of EGCS.