AIX is still ppc64be. That and s390x are the only big-endian CPUs I can think of which aren't end-of-life, which I think is going to be an increasing maintenance burden over time for IBM alone.

I think they see customers wanting to have the flexibility to move to ARM and this is the fastest way to say they support ARM workloads. Maybe this is a path for IBM to eventually use ARM chips down the road, but I see this as being more about meeting customers where they think the demand is today rather than an explicit guess for tomorrow.

ppc64le has other machines. Raptor off the top of my head, but there's also that weird notebook project that seems to be talked about once every few years and probably won't ever happen and some pretty cool stuff in the amiga space (I don't know if that's strictly le but power is supposed to be bi-endian)

ARM does not erase the compiler and toolchian tail IBM has dragged across two niche arches for years.

Legacy apps on s390x do not move because IBM put out a press release and IBM does not get fatter cloud margins by joining the same ARM pile as other vendors. Mainframe migration is not a weekend project. "Easier" usually means somebody signs a six digit check first.