This isn't a counterargument as such, but you could apply similar logic to, say, banks and COBOL? I'm not in banking, but I imagine there are many good reasons to keep writing and maintaining COBOL, and these companies may have similar stories to tell about Ruby.
COBOL and many legacy systems have the nasty problem of everything being inter-connected: a textbook example of spaghetti code.
So in such systems, in order to even begin properly, you have to rewrite a sizeable chunk of the legacy code before you could even showcase a first demo / MVP.
Needless to say, the relentless culture of our modern times makes the businessmen never approve such projects.
