ARM is fine for what it's intended for, but a fire-breathing high performance general purpose CPU suitable for weather forecasts it is not.
HPC teams already use heterogenous hardware mixing x86 with GPUs because x86 hardly shines at parallel vector work. While x86 delivers great single-thread performance that necessarily the most important part of HPC. ARM chips already come with optional hardware acceleration packages, throw in FPGAs and GPUs and, at the right price*, the HPC crowd will be drooling.
* If you look at some the biggest HPC installs it's obvious that purchase price is not that important. Increasingly, it's important to have something that doesn't need its own dedicated power station.