Your first 'graph is what I'd been taught in economics, backed by arguments of marginal productivity and marginal costs. I no longer find it credible.

Among other factors, market prices for goods are not fixed, and in many ways, they act somewhat as a "hairpin bend" effect -- there seem to be activities which settle at a given price or ratio regardless of other effects. One of the more notable arguments around this is Baumol's cost disease: there are products (in particular service-heavy ones) for which wages have risen despite no increase in productivity, due to the rationale that the talent for such work would otherwise move elsewhere. A string quartet, regardless of technological improvement, requires four performers. (Though recordings, broadcast, and potentially AI-generated alternatives might be developed.)

There's a certain parallel with Amdahl's Law: limits to which any computing task can be made more efficient through parallelisation, given the non-parallel component of the task. If 1/5 of processing time is non-parallelisable, then the greatest efficiency possible, even with infinite parallel processing, is a reduction in time to 20% of the original.

For a non-technologically-enhanceable task, if labour comprises 20% of the original cost, then it ultimately rises to be all of the expense, and the maximum possible cost reduction is 20%, even with infinite efficiency improvements in all other factors.

More generally, what economics does is shift utilisation amongst inputs based on the apparent cost of those inputs, relative to their minimum required contribution to production.

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Another line of argument stems from Adam Smith (who, incidentally, fails in the above argument), who describes the factors contributing to high or low wages above the minimum required for survival:

The five following are the principal circumstances which, so far as I have been able to observe, make up for a small pecuniary gain in some employments, and counterbalance a great one in others: first, the agreeableness or disagreeableness of the employments themselves; secondly, the easiness and cheapness, or the difficulty and expense of learning them; thirdly, the constancy or inconstancy of employment in them; fourthly, the small or great trust which must be reposed in those who exercise them; and, fifthly, the probability or improbability of success in them.

Smith omits a sixth factor: labour organisation (though he addresses that elsewhere).

Traditional factory work represents at least several of these factors: it's often disagreable (hot, noisy, dirty, dangerous), requires skill, may be inconstant (factories may start or stop operation based on external factors), and skilled workers may well be in a position of trust as regards capital, material inputs, reliability, and work outputs.

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Smith also, in Book 1, Chapter 8 of Wealth of Nations describes at length the conditions and consequences of falling labour demand. It's a bleak picture. One I would think we'd be well advised to avoid if at all possible.

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He also makes clear that the bare minimum acceptable wage is one that will provide for a worker and their family, as otherise it's not possible to sustain the next generation of labour. If you cannot operate profitably whilst paying a living wage, then what you are operating is not a business, it is a charity, on behalf of the owner, with contributions withdrawn from the life-force of the employees.