>So IBM wants to switch to ARM without making a too big fuzz about it.

That was my first thought too, but it does not make sense, because if IBM would sell ARM-based servers nobody would buy from them instead of using cheaper alternatives.

As revealed in another comment, at least for now their strategy is to provide some add-in cards for their mainframe systems, containing an ARM CPU which is used to execute VMs in which ARM-native programs are executed.

So this is like decades ago, when if you had an Apple computer with a 6502 CPU you could also buy a Z80 CPU card for it, so you could also run CP/M programs on your Apple computer, not only programs written for Apple and 6502.

Thus with this ARM accelerator, you will be able to run on IBM mainframes, in VMs, also Linux-on-ARM instances or Windows-on-ARM instances. Presumably they have customers who desire this.

I assume that the IBM marketing arguments for this are that this not only saves the cost of an additional ARM-based server, but it also provides the reliability guarantees of IBM mainframes for the ARM-based applications.

Taking into account that today buying an extra server with its own memory may cost a few times more than last summer, an add-in CPU card that shares memory with your existing mainframe might be extra enticing.