No economist worth his salt would deny the superiority laissez-faire over a steered marked.
A laissez-faire market is still steered - "market forces" are for more than the individual preferences of "standard" consumers. I mean this as gently as possible, but your statements point towards a caricatured, dichotomous portrayal of the two, when really they're both points on a scale.
The difference is that the state or some central authority is not steering the market.
The structure of the market also steers the market - it's not possible to disentangle the two. Again, a matter of degree, not red = laissez faire, blue = controlled market
But it is undeniable that when a decision is made politically, which ignores market prices, it will use resources less efficiently than the free market would have.
Actually, private sector waste is commonplace - what you've said here is a popular myth, but a myth all the same. What matters is the quality of the incentive structure and the quality of the actors within that incentive structure. The profit/not-go-out-of-business incentive, with people who can think in terms of strategic, long-term self-interest, works very well - but very, very few humans, senior managers and CEOs included, are good at thinking about long-term self-interest strategically. The longer-term the goal/benefit for society, the less effective that simple, short-term pricing signals are in terms of getting efficient results. Climate change is perhaps both the largest example of this, but there are plenty of others. Mentioning that example also highlights the "tragedy of the commons" - the most obvious situation when simple free-market market structures fall down.
And even if we were bothered enough to say that this does not account for post-Keynsian economics, that is a valid point but
post-Keynesian economics began in 1936.
Keep in mind that while a post-Keynesian understanding of economics obviously is something that occurs after the Vicky timeframe, people during the timeframe undertook economic interventions that could be understood as Keynesian, even if not labelled as such.
Bear in mind that economics is very, very complicated, and even the best in the field don't understand everything - your characterisation of economic understanding asd "done and dusted" here is at odds with the vast majority of the economics writing that I've come across (and at odds with all of the stuff that was of any quality). It's not as simple as 'we know how it really worked - this is what should happen' - economics is not that simple. Indeed, the discipline is only now, after well over a hundred years, starting at a broad scale to treat humans as if they aren't rational economic maximisers - something that was plain to see back in the days of Adam Smith (Adam Smith actually wasn't as silly to think humans were rational economic maximisers either - he's often misrepresented by people that would benefit from reading his work).