I’m a retired IT consultant, or 45 years in the business. Way back when mainframes were the size of suvs there were no viruses and the only way to hack one was to be inside the same room and physically damage a computer or destroy code. There were no outside connections, except later linking mainframes with pre tcp-ip like protocalls. Not until tcp-ip was fully developed along with the internet when computers (mainframe, midi, mini and personal) were linked and viruses were discovered and remotely loaded with destructive execs. As long as computers are linked and accessible remotely and/or not in very secure buildings they will be hacked. It’s a challenge to some who cannot control their desire to “beat” the security, not to mention some monetary gain.
Now Mainframe security is one of those hidden but in demand skills.