Look to the future and imagine the internet of things ... yet all those smart devices relay on the big data consuming services of their proprietors.
Why are mainframes so hard to replace you ask yourself. Versatility is my answer.
Sure now, how can a machine weighing tonnes be agile and able to adapt to an ever changing world?
The answer is within that ever changing world itself, ask yourself what is a mainframe exactly?
I see mainframes as ultra concentrated bulks of technology, not measured in mips or dry-stones but in simultaneous multi processing threads.
Rather described in technology level instead of architecture and operating system.
On it's own every mainframe is a supercomputer ...
No I'm not a mainframe operator, though sometimes I wish I was. Once I've had a summer job caring for a GCOS/Bull system in night time.
I enjoyed the smoothness and sheer power of it. It was the start of my carriere in IT.