[Thiez]

> Especially since one huge strength of Rust is its C interoperability, one would imagine that incrementally replacing or extending parts of the legacy tools would be a much more sound approach.

When it is unlikely that the maintainers of existing tool written in C would accept pull requests that add Rust as a dependency (and I imagine this would usually be the case), why wouldn't you start anew? It also allows easier experimentation on the architecture of the original tool, and breaking compatibility. For example, ripgrep isn't a drop-in replacement for grep and the author never intends it to be, so forking grep probably wouldn't have made it easier to develop.

I think the difference between a learning project and a "real" project is mostly how many people end up using it.