I'm about as old as you can be and still be considered a millennial and I've only run into mainframes twice in the wild, at my 1st and 3rd jobs. The 1st was tha

I'm about as old as you can be and still be considered a millennial and I've only run into mainframes twice in the wild, at my 1st and 3rd jobs. The 1st was that way because they were leasing the software written in FORTRAN by 1 guy in the 70s and doing a couple very small things on top. All the core edits had to go through that one guy (this was ca. 2009 or so), it was basically a black box with a 6+ week turnaround for even minor changes. 3rd was for a warehouse management system and logistics shop that was so old they had multiple full-time employees whose job title was "Computer Operator" and basically just babysat the mainframe - zero programming, sysadmin, or networking skills, I still have no idea what they actually did but whatever it was it was enough for 3 or 4 full-time people. Despite the Computer Operators, the advertised pros of the mainframe are often very little maintenance and being able to write code and let it run for months or years without interruption. I've heard the performance is also very very good.