I was unaware of alternative IRs for julia (albeit I started learning much more about compilers once I left the julia ecosystem). What other IRs is this supposed to replace? And why not just go with MLIR directly?

Julia has roughly 2 different IRs that live pre LLVM (one for type inference, one for Julia level optimization). This is a proposed replacement to both of these. The reason not to go MLIR is that writing Julia is a lot nicer than writing C++.

MLIR pulls in a lot of interface complexity that we don't want or need for pure-julia end-users. That said, it is obviously and deliberately close to MLIR as a representation since people do want to be able to interface with MLIR (e.g. in Enzyme), so I'd like to facilitate easy conversion back and forth.