They are distinct issues, to be sure, but, then again, they are part and parcel. Properly implemented, a language with distributed computing primitives buys you

They are distinct issues, to be sure, but, then again, they are part and parcel. Properly implemented, a language with distributed computing primitives buys you single address space concurrency: it's the degenerate case of a cluster. No matter what, a single machine is going to be less in terms of total computational capacity and fail-over ability. Languages that optimize for single machines, however nice, are something of a tease.