Where does the myth of irreversibility come from?

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Conservation of Energy (First Law of Thermodynamics) means there are quite strict limits on energy conversions.

The worst machine returns 100% of the energy given.

Should any process converting energy yield more total than it started with, or less, that would be creation or destruction of energy.

The worst device with the least efficiency still obeys this fundamental law, at least for all processes currently known to man.

Inefficient devices still return all the energy they are given, in one form or another. Efficiency is almost meaningless to discuss from the point of view of the Physics of energy. Its always 100%. Anything else means energy is being created or destroyed.

Energy is only rearranged, as far as science has shown so far.

(The First Law is unproven, it is the consensus from observation and many many measurements.All tests so far are consistent. Some theories such as the Big Bang theory have at their core energy creation. Others, like dark energy causing an accelerating expansion of the universe would require energy to be continuously created in massive amounts, but as yet no tests have been devised to prove or disprove those theories. As the First Law is in the strictest sense unprovable, the scientific method does not rule out theories just because they contradict the First Law.)

Mankind has an infinite capacity to believe contradictory things.

Physicists will tell you no process can destroy energy. Some of them will also tell you that irreversible engine's, which is anything but the Carnot Cycle (they say), are less than 100% efficient.

Where does that come from? Lord Kelvin's theorem in "Reflections" states no <as yet unknown> engine can do better than a reversible engine, or the consequence would be a means to create energy from nothing. (This would be a process more than 100% energy efficient).

Human nature flips that all <as yet undiscovered> engines must be worse. (I mean, what are the odds they are the same energy efficiency?) It seems that this flawed logical inverse is the root of the myth about the Carnot Cycle. To the contrary, and ignored by history and physicists alike, Lord Kelvin goes on to say "any two engines, constructed on the principles laid down above, whether steam-engines with different liquids, an air-engine and a steam-engine, or two air-engines with different gases, must derive the same amount of mechanical effect from the same thermal agency". So no "irreversible" engines are possible, according to the giant of thermodynamics.

The correct logical converse, is that if any <as yet unknown> engine were worse than a reversible engine, it would be less than 100% energy efficient, and if so would indeed be irreversible.

So, irreversibility requires destruction of energy, which by exhaustive test appears impossible. And vice versa, destruction of energy requires irreversibility.

Reversibility of vapors requires conservation of energy, and conservation of energy requires reversibility of vapors.

Since no engines are known which destroy energy, all known engines are reversible engines. There is no variation of energy efficiency based on engine Physics. Rather, efficiency is comparing the desired result to the actual result, not comparing the total energy return.

Efficiency is the sole domain of design, construction and materials of a device.

Efficiency is also inherently a measurement from the human perspective, a subjective measurement.