mainframes and their operating systems were designed from the ground up to handle hundreds if not thousands of users concurrently. Raw horsepower crunching numbers, like bank account statements. Operating systems so "limited" to the average hacker that no one has ever written a virus or trojan for any of them. No mucking around with a GUI - just a plain command line, batch command files, etc., keep the lights running 24/7/365.

But any PHB can replace their mainframe with dozens, if not hundreds of servers running Windoze, and then paying for all of the networking people, sysadmins to keep it all running, etc. Get Linux and user fewer people, but you have to figure out how to port all of your applications over to new software. At least COBOL has been bulletproofed over the past 4 decades.