Bash or csh don't have to know about "drive letters", just like they don't need to know about /vol, or /usr. It is just part of the file path and any file path that the OS understand is fine for bash or csh.
As for" forward" [sic] slashes, c:/xyz is a valid path for windows. Even if it weren't, the completion logic in at least bash is fully pluggable so it would just* need a completion module to support backslashes, besides recompilation, ironing out niggles that would creep in and bug fixing.
Even if bash completion weren't pluggable, Microsoft could edit the source code to support backslash file completion. So even after making multiple wrong assumptions in your argument's favor, your argument is still wrong.
*If bash code turns out to be non-portable, the work will be a bit more.