That labor gives things a significant portion of their value is not a self-evident fact, so any reliance on that being a self-evident fact is the first step down the wrong road.

Capitalism has nothing to do with SNLT. Under capitalism, a thing gets value from being the best available substitute for not having something with that thing's utility. The better mousetrap has value because it is the best available substitute for rodent infestation. . In this context, labor can destroy value by making the most useful mousetrap too expensive and, therefore, not the "best" substitute for the circumstance it would remedy. The value comes from the invention that makes the labor socially necessary, not from the labor itself (other than Edison's 99% perspiration).

I have never heard a Marxist say anything about the value of risk-taking, which is the essence of capitalism. The capitalist estimates what labor will be socially useful (if not necessary) and takes the risk that the labor will prove not to be so. Should that risk, and the service of deciding what labor should be attempted, not be compensated out of the value created? And, if so, isn't the decision to allocate the labor itself socially necessary labor and a source of value?

The whole labor/capital distinction makes no sense to me. Everyone from capitalist to janitor uses their brain to some extent to cause a useful thing to come into being. Class distinctions arise from bargaining power, which arises from scarcity of critical talents. There are obviously political barriers to mobility, but they are not really what "capitalism" is about. Inheritance is a complex social mechanism. At one time, good union jobs were inherited, too.

Ironically, I share your view that we are (or were, pre-CoVID) on the cusp of a post-scarcity economy. Jeremy Rifkin was a college classmate, and I have read his on the zero-marginal cost society. I am a believer in Modern Monetary Theory. But I do not believe that capitalism is the wrong way to allocate even abundant resources. Wasted effort is still a bad idea. Rather, government must channel capitalist activity in the way child labor laws and anti-trust laws have channeled it. The idea that the whole system must be replaced is moralistic resentment on stilts. A tweak here or there is all it would take.