Virus scanners cannot generally detect things which aren't viruses. Your batch file isn't a virus.

Anti-virus programs use signatures to identify known viruses. Because viruses spread so much, new viruses quickly fall into the hands of AV manufacturers, who add them to their database.

Programs which don't self-replicate cannot spread quickly enough to turn up in AV databases. However, the AV people do add a few non-virus signatures, typically for common Windows "Remote Access Trojans".

Note that it's generally impossible to know what a program will do without running it (forever), according to some mathematics proved in the 1950s (The Church-Turing Thesis to be precise).

Therefore, a virus checker is going to have a hard time telling whether a program will erase Windows.

Also, your simple logic bomb wouldn't work. Windows does not let you delete files that are being used, and the Windows directory is definitely in use.

However you could trivially delete enough files to make Windows stop working.