Disclaimer: I do not work for IBM. I also can't stand Mainframe salesfolks who have repeatedly blown me off when I try to bring them in on some accounts. I am no IBM fan for many reasons, BUT:
The beauty of Mainframe is that it can act as a big cloud with centralized management or it can act in a highly-scale-up system as well.
The ability to run literally thousands of Linux instances with a physical (energy, rack space) and support footprint in the fractions of what it would cost to manage several hundred VMWare boxen.
Workload management and on-demand ergonomics of CP(U) and memory resources is simply decades ahead of anything else. This is very important when you are overcommitting computing resources, and running any cloud services company, this would be crucial to your margin.
Stepping down from Mainframe, even AIX can run circles in vertical and management scalability around other computing
Remember the cloud is no different from colocating some slabs, just at an extreme price point for the consumer to adopt as well as the ability to remove the liability of most systems management. Ask yourself this, if you were to create a new Amazon AWS-style service, what architecture would you choose from the context of your P&L?