Why would anybody need actual x86 or ARM or any other specific low-level instructions set on a server? I doubt server side developers program in assembly often. As long as there are a performant JVM, a HTTP server and a database management system available, everything is possible and almost nobody (but system programmers and also those who pay for the spare parts) really have to care about whether it is x86 or what.
Of course you have to care about architecture. ARM has been in the making for a long time now, and I can only imagine how far behind z is in terms of software support. To succeed, IBM would have to get Linux, Postgres/MySQL, Nginx/Apache, Java/Python/NodeJS, and the rest of the whole modern x86 ecosystem to work on z—that project would never be done. Or they could build their own, different tools—but they kind of already have, and why would anyone switch to those? To catch up with the other cloud providers, IBM needs to start with what everyone's already using—x86—and use their capital to build an advantage from there.
How will people use it?