If so, should we assume that thinkers like Plato were giving cover for a tradition that had its route in economics/scarcity? Something else?
The idea that Sparta killed unfit children is indeed famous, thanks in no small part to the movie 300, which states it as fact. But Sparta almost certainly did not do this. We have only one source for this supposed practice, which dates many centuries after Sparta's heyday. I've previously explained in detail why we should not take this source at its word. Recent scholarship has cast doubt on it from multiple perspectives - its late provenance, its doubtful context, its vagueness, the lack of corroborating evidence (and even the existence of contradictory evidence), and so on.
Similarly, there are a few passages in Plato and Aristotle that have often been cited as evidence that the Greeks routinely exposed babies who were perceived as disabled, but the case they make is weak. In a recent article, Debby Sneed has shown^1 that these philosophers are describing their ideal world, not the real one in which they actually lived. A few elite men arguing that disabled children ought not be raised does not mean that the Greeks generally followed this rule. In fact, it suggests the opposite: that reality was not as these philosophers wished it to be. Sneed goes on to offer various kinds of evidence (literary and material) showing that disabled children were indeed often raised to adulthood, even if it took special effort to make this possible.
In other words, far from a well-attested ancient Greek practice, the exposure of disabled children seems to be something that a lot of modern scholars and laypeople have assumed to be common because it fits their assumptions about life in premodern subsistence economies. The shaky evidence from philosophers and moralising authors is good enough only if you start from the premise that infanticide was probably common. Many would argue, and many have indeed argued, that it "just made sense" for ancient people who regularly faced critical food shortages to avoid raising children who would struggle to make an economic contribution as adults. Plutarch and Plato simply supply confirmation of what we already "know".
But this assumption does not reflect historical reality. We have plenty of evidence of ancient adults with disabilities treated as full members of society. We have good evidence of special arrangements made to support people with disabilities. Moreover, the evidence (such as it is) for systemic infanticide in places like Sparta applies only to the leisure class, showing clearly that even if it was a real practice, economic concerns had nothing to do with it. There are serious risks involved in citing economic "common sense" to cover for explicitly eugenic practices within wealthy ruling elites.
Meanwhile, the sacrifice of children in religious rituals is a very different subject, about which I don't know much. But I do not think it should be mentioned in the same breath as the supposedly systemic killing of disabled children. These religious rites were presumably not intended as a way to police the food economy, purify the gene pool, or any other structural factor that has been raised to argue that infanticide was normal and common.