> The problem with the filesystem could have been solved by optimizing the Windows kernel

Over time this would tie the Windows kernel’s requirements so that they matched the Linux kernel’s due to expectations from WSL1 users. This of course is a bad idea for any engineering organization - you will have requirements imposed on you that don’t mesh well with your other non-WSL users and you also have no real sway over Linux governance. This would lead to the Windows kernel either becoming a clone of Linux or serving at least one set of users poorly.