The idea of r/K selection theory applied to humans is that a population could be r-selected for fast Life History - such as rapid sexual maturation and high fertility, with relatively low levels of parental investment into each offspring;
or else K-selected, for a slower and more long-termist Life History - fewer offspring with more gradual and delayed development, and investing more resources per child (with the aim of generating more cognitively specialized adults).
(In a nutshell, and approximately - r selection is for quantity, K selection is for quality. In some environments only 'high quality' offspring - making which requires longer development and more resources - are able to compete successfully with other members by aiming for narrow niches requiring particular qualities.)
But r and K are not opposites. Nor are they reciprocal: to reduce one does NOT mean to increase the other.
Because 'selected-for' means 'specialized-for'.
And specialization implies adaptation.
Therefore to be r-selected is to be specialized-for r - this means that an r-selected population has evolved a suite of adaptations which together enable it to become better at rapid and fecund reproduction.
And to be K-selected is to be specialized in producing offspring who have a better chance of themselves reproducing in a context of more long-termist Life History.
And to lose long-termist adaptations of K-selection is NOT thereby to gain short-termist adaptations; and to lose short-termist adaptations is NOT thereby to gain long termist adaptations.
So, in the modern world, the selective regime in the West has resulted in sub-replacement fertility for K-selected populations, and this will indeed destroy-K selected adaptations; but the resulting population will NOT thereby have r-selected adaptations by default - the population will just lose adaptiveness!
(Loss of adaptiveness is - more or less - disease. Mutation accumulation is disease. On average, disease does not benefit adaptation in any way.)
And a selective regime (such as the modern world) which reduces child mortality from approximately 60 percent to about 1 percent, allows mutation accumulation in r-selected populations.
Mutation accumulation will NOT increase r-selected abilities, it will NOT improve short-termist adaptations - but the opposite: these r-selected populations will lose their Fast Life History adaptations, as these specialized attributes enabling a fast Life History will be damaged by accumulating deleterious mutations.
So the modern world is NOT becoming more r-selected - it is become less K-selected AND less r-selected: the modern world is becoming less adapted all round.
This is concealed (temporarily) by the expansion in numbers of the previously r-selected populations- which is enabled by the massive reduction in child mortality rates and an increase in longevity - and an illusion of r-adaptedness - but in fact these populations are LESS r-adapted now than they were before their populations began to expand.
If r-selection is summarized as specializing for quantity and K-selection as specializing for quality - then both high quantity and high quality are adaptive products of evolution. And, destroying quantity does not improve quality - also destroying quality does not improve quantity.
So modernity does NOT increase r at the expense of K - instead, modernity destroys BOTH r and K adaptations by means of mutation accumulation.
What results is that humans as a whole have lost adaptations, both short-term adaptations and long-term adaptations: the human genome has been damaged.
Note: The above idea comes from Michael A Woodley - to whom credit should be attributed; but any errors or inaccuracies in expression are my responsibility.