If anyone is looking for some good discussion about mainframes, there was a lot of good content that came out in this thread:
https://news.ycombinator.com/item?id=4446880
Someone asked a similar question a few years ago and there was a good discussion on HN about it here.
My father's entire career revolved around IBM's TPF mainframe systems. Airline Reservation Systems, Credit Card Processing, and the IRS use it for extremely high volume processing.
If anyone is looking for some good discussion about mainframes, there was a lot of good content that came out in this thread:
https://news.ycombinator.com/item?id=4446880