But, one way of looking at it, that's just ("just") an implementation issue. No ideas fundamental to Nix would have to be changed to find a faster way of implementing derivation isolation, hopefully any solution to problems like this could be transparent to people who had written build systems in Nix and not require significant rewrites...?

Even so, incremental builds wouldn't be an all-or-nothing scenario. A project could be divided up into a number of separately buildable partitions, each built as a separate derivation. Deciding how granular to make your incremental..ism would just be a tradeoff of overhead vs. amount of rebuilding saved.