People have been asking this question for decades, and the answers haven't changed - 1) mainframes do some things very well in terms of security and stability a

People have been asking this question for decades, and the answers haven't changed - 1) mainframes do some things very well in terms of security and stability and 2) porting software is expensive. In the mid '90s I was working for a company that bought the "Mainframes are old technology and we're not an old technology company" line (those industry broadsheets should really be kept away from management) and set out to replace the old reliable mainframe with OTS Unix machines. We got the answer to the OP question and we got it good and hard.