Thanks for asking.
I don't think of the law of diminishing marginal utility at all. Never heard of it 'til you asked me. I'm neither an economist nor an intellectual nor do I want to be, but I get what you're saying, though only in terms of how this law purportedly works.
What is this 'argument' that 'this process should cause a natural equilibrium in the supply, demand and cost of cars' suppose to convince me of?

The point of the post is that income for labour is a social tenet that most of us just take for granted and accept. However, jobs are being replaced by technology and the time is not so far away when the demand for labour will be a fraction of what is was when this tenet had some validity.

The upshot is that (in my opinion) we will need to restructure how we think about labour as the primary source of income for the majority, if society is to meet the needs of our new situation and we need to do it fast.
Yet most politicians are still talking about creating jobs at a time when in the long term, that direction has lost validity.