One major hole in the theory that the relationship between the twice born castes, particularly Brahmins, and the remaining castes was primarily of a parasitical nature is that in pre-modern times, such relationships would mean a significant advantage of the parasitical caste
in terms of reproductive fitness vis-a-vis the others. This would mean that if such a system existed for thousands of years, over time the parasitical castes would preponderate over the non-parasitical castes. But if we look at modern numbers, we see that twice-born castes are
barely 15-20% in most of the big North Indian States, which doesn't make sense. If Brahmins, Kshatriyas and Vaishyas were mostly living off parasitically on the rest of the population then the reproductive edge they would have gotten would have meant that they would've certainly
constituted a vast majority of the population over time. Even if we were to take into consideration that upper castes have undergone a fertility transition much before other castes in the modern era, it is doubtful that even in the early 20th Century they were more than 30% of
the population of the Gangetic Plains. Instead of seeing the Varna-Jati system of social predation and parasitism, I think it's probably better to understand it is as a system of conflict avoidance and efficiency maximisation through specialised skill sets. This is not to argue
that Varna-Jati did not probably cause a trade off by choosing conflict avoidance over short-term gains in efficiency, but it seems likely that specialisation probably made up for a fair bit of that loss in productivity. Also, this thread is not presenting an argument in favour
of strict Varna-Jati compartmentalisation of labour in today's modern economy, since labour allocation today moves much more rapidly than is possible for such a system to continuously align itself with.