Do humans really miss anything important in their diet when they have to substitute fat with more carbs? As in being a challenge to the multigenerational surviv

Do humans really miss anything important in their diet when they have to substitute fat with more carbs? As in being a challenge to the multigenerational survival of a community, not as in missing or on national health gains? A population that is protein-starved will be in real trouble, no matter how plentiful everything else is. I think it makes perfect sense to give special attention to the protein source, even if it's also a major energy supplier with the fat that comes along. But of course it's well possible that the focus on protein is mostly coincidental, as described in GP's excellent post, and not so much motivated by substitutability. But I'd argue that it's a convenient coincidence.