The mainframe is a different beast.

Can you change physically change CPU, Memory, and internal components without any stop/delay?

Not only the Mainframe is Very robust it was made not to stop.

I used to work on a Data Center with Sparcs, Intel, Blades servers, and Mainframe.

The only time I saw the Operator with Fear was when he had to turn off the Mainframe because of electrical maintenance. The last time he did shutdown the Mainframe was 15 years ago.

VMs? The mainframe has it since the '60s.

IO? The mainframe is a BEAST on IO

Backward Compat? IBM guarantees that your code from the '40s will run today.

The whole research in mainframe is to Be a powerful beast with high availability and safety.

If you may check more about it, it is a part of Tech that is very beautiful yet.