I recall there was also an issue with how paths are treated in NT. I don't fully recall, but I think NT paths are parsed by the kernel early on, and the whole kernel operates on "cooked" paths. there was some major performance implications this had for WSL1 in addition to the filter driver architecture.

I also don't remember why they couldn't just bypass the filter stack for paths in a certain volume - WSL2-like I/O on WSL1 - but there must have been a reason.