The "init" process is just a stub used by WSL, it is not a "real" init system. And Cygwin also allows the creation of multiple users within the Cygwin environment, doesn't it? It's just a matter of populating /etc/passwd. If you want to use WSL1 with a single user then you could just set the default user to root.
msys2 mostly cares about running your textmode software.
For example, someone talked about processes. Here's all that I see in msys2:
# ps xwau
Most of the time, I don't need to access Windows processes - and if I do, data can be exchanged through a file.