Lots of things were a complete rewrite of vi. Vi was a complete rewrite of ex. ex was a rewrite of em, which was in turn a rewrite of ed, which was a rewrite of qed.

Trying to rewrite vim is the wrong approach. Trying to replace it in a backward-compatible way, the way vim did to vi might received a lot better.

Vim started as a Vi clone for the Amiga and later grew to include more features and target more systems.