"Most really large systems run on big iron. No flavor of Unix, and certainly not Windows"

Really large corporations DO run a Flavour of Unix. (IBM AIX is huge in the Banking industry.) I think they qualify for large. Just a few thousand dumb terminals.

How about the Railways ? They are kinda large. They run IBM and AIX. (the ones that I have personal knowledge of)

And the Steamship Lines ? With worldwide offices ?

I can go on, if you like to medium sized companies.

And, (chuckle) ever seen the OS/2 boot disk for a bank machine ?