A racked z-Server hosting cloud(s) can be an on or off premise aas like any other platform and can scale out the same way. On/off premise a just choice based on your business model. You can even have both, and a mixture of platforms.
I don't know why you think zSeries cannot host multiple VM's or containers because they can. Virtualisation in the mainframe is nothing new, it's matured over decades at physical and software level. Linux can run natively or under VM, as can zOS, at the same time if you wish using ultra-fast interconnect within the processor to talk to each other. All this with 100% pervasive encryption, include in-flight. You seem to think z-Arcitecture is out dated, but I'd say it is your understanding of it that is out of date.
Your point about a billion customers is moot as I suspect there are still relatively few companies servicing that number and it would be a challenge for any platform. And it's not just customer numbers that is relevant, it's the transaction rate that counts, along with network bandwidth, and how many applications interact to name a few.
In terms of agility that has less to do with the platform and everything to do with the company structure and culture, standards, and available talent.
I am not an architect, but I have had to suffer the whims of architects who too often work at a theoretical level and have no practical operational experience.
Biting the hand that feeds IT
