I'm still learning about this concept, myself, but my over-simplified understanding is that humans are considered to be "k" species, but some are more "k" than others. This is crucial for my idea, at the end of this...
Is there a "point of no return?" Well, that's the tricky question, and why I bring up Koestler...my Objectivish leanings, with the emphasis on free will and choice, say that adaption is possible, depending on the context. I do believe in free will. That said, there's something to be said about that Jesuit "Give me the child for his first seven years, and I'll give you the man." ( I think Molyneux has said something about the first 5 years, recently, on the Manson podcast.) Changable, but not infinitely so. That's where, for me, Koestler meets Jung (Jung did influence Koestler, re synchronicity. Here, I'll add that I personally find value in both of them, but reject the more "woo" explanations of Jung, as well as his Kantian influences...again, me being Objectivish...). Psychological complexes develop that shape an outlook, far more than genetics, in my view.
Stanton Samenow talks about this, in regards to criminals, malignant narcissists, etc, in INSIDE THE CRIMINAL MIND, about how certain thought patterns and mindsets become so entrenched that it is not genetics per se, but those defense mechanisms, that make certain people resistant to change. (And they have to WANT to change, to begin with.) Usually, he says, in order for that to happen, they have to become so disgusted with themselves, but that it's also rare, that they would rather fight to the death to protect their view of themselves than change. (Along the lines of the "first 5-7 years" idea, Samenow also urges early intervention for troubled children BEFORE their thought patterns can crystallize to the point of no return...)
But back to the r/K idea, which scientifically is about breeding and such, there's the idea that environmental factors can force change, i.e, resources drying up, etc. But we are more than animals, depending on the chance of nature providing; we are conceptual, and can recreate and guide conditions via farming, agriculture, etc. And it makes us not just a "predator" or "prey" species, but a "trader" species. (Not just being objectivish here, though I certainly am, but also pointing to something like Jane Jacob's so-callled "guardian" and "trader" syndromes (so-called, because she uses syndrome in a different sense than the common negative connotation). But if we are traders, we can also interfere, for better or for worse, with Darwinian progress via social programs, welfare statism, etc...
That conceptual factor is demonstrated with the idea of the trickster. (To invoke Jung again, who wrote about such characters...) Tricksters often advanced change, often by accident, by emulating different animals, and adapting their traits and behaviors, to various effects. There is where I think the r/K theory, scientifically, interacts with the metaphorical social application of the theory to humans (if we truly are a "k" species, with some being more K than others...) And biologically, women can only have so many children at a time, compared to reptiles, which can birth hundreds, and generally are required to raise them, unlike reptiles, which are born ready-to-go. And yet, in the cases where r/K theory is applied to humans, we have women who have as many children as they can, for various purposes, and sometimes abandon them. It's not just the genetic component, but the conceptual adaptation of other species behaviors (perhaps based on environmental choices or limitations faced similar to those of "r" species animals, that makes it SEEM as if some people are "r" and some are "K"...And that's where the Koestler "triune" brain idea intercepts, because we do have those more primitive brain structures that all animals share, but with the mammalian and cortex in addition. So the conceptual adaption of "r" behaviors in a "K" species becomes a unique phenomenon, and that's where the metaphor/comparison breaks down and humans, and becomes something more...
That's my theory.