Myth: social engineering is impossible.
Reality: social engineering is the easiest engineering discipline.
Classical physics is in fact a special case of quantum physics. In large numbers, all the weird randomness cancels out and you're left with a bit of algebra. Similarly, predicting a single person (especially at range) is extraordinarily difficult, but with large numbers the divergences average out to be particularly simple.
Helpfully, the Russians checked this hypothesis for us. The KBG ran around executing elaborate plans engineering controlled demolitions of other societies. Sociology is so easy they didn't even need to prototype. It just worked.
The myth comes from progressives, as modern myths are wont to do.
Since sociology is easy, the negative effects of progressive reforms were all predicted in advance. Minimum wages killed employment, especially among the poor. Affirmative action led to the 'beneficiaries' becoming violent, degenerate fops. 'Emancipating' women destroyed the family. Etc. etc. If you had to wait for the proof of the calculation problem to know that communism would be a catastrophic failure, you were an idiot.
But, for obvious reason that I'll nevertheless belabour, progressives lie about it. (Recall the difference between lay proggies and the leadership.) They do these things because it benefits them. Since they are perceived to be evil and are in fact comically destructive, proggies are can't be upfront about it. Nevertheless, you can tell the policies are working as intended because they're never rolled back.
Further, for the same reason, you can tell that for the most part it's a plan and purpose. While certainly there are prospiracy aspects to the progressive parasite regime, for the most part sociology is easy so they plan and then it just works.
When minimum wage laws destroy the dignity of lower class neighbourhoods, they become dependent on the government for survival. To paraphrase a certain fungous insect, if you own a man's livelihood there's one thing you've certainly bought - his vote. Proggies outlawed marriage because married women are far more likely to vote on the right. Proggies opened holes for illegal immigration because, even setting aside the vote thing, a suspicious, distrustful population is far easier to divide and conquer. Indeed the whole immigration thing comes with its own built-in division. Saves time on cutting new ones. Letting homosexuals out of the closet destroys male companionship, thus men must turn to the government. In case you think this is just a gay coincidence, in areas where sodomites can't be used to dismantle male camaraderie, heavy-handed persecution is applied. If military history can't be made fruity or boring enough, then funding for curricula and departments is simply cut, and amateur societies are brought before kangaroo courts.
If it were some kind of blind groping there should be policies that accidentally harm progressives. These failed initiatives should be rolled back. In practice, Cthulu always swims left. There have been rollbacks but because of overreach, not because the policies ever threatened progressive hegemony.
The peasants have the attitudes the progressives want them to have. If they don't behave exactly as progressives want them to, it's due to failure of will. The progressives prefer to be the most hip and fashionable in any case, so this is more feature than bug.