I think the concept of "sub system" is quite integral to the NT kernel. Might have been easier if they had stolen the name from user-mode linux - which might be more suitable for wsl2 in some ways... But that was/is a quite different type of project:

https://en.m.wikipedia.org/wiki/User-mode_Linux

See "Win32 environment subsystem" and "OS/2 environment subsystem" here:

https://en.m.wikipedia.org/wiki/Architecture_of_Windows_NT

Then there was coLinux: https://en.m.wikipedia.org/wiki/Cooperative_Linux

Maybe they should've called it LINE (after WINE)....