I would argue that this is largely because the systems you mention do not have the requisite (hardware or software) functional abstractions required to make them mesh nicely with functional or declarative control* code. Backus here is writing from a hypothetical word in which that is not a problem.
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* a dangerous word in and of itself in such a conversation, because of its imperative significance.
Wouldn't that just place the burden of providing those abstractions on the hardware implementer. In essence, are we not just pushing the problem down the stack?
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* a dangerous word in and of itself in such a conversation, because of its imperative significance.