So, here's the thing about reductionism used in economics and econometrics. Economists do use (as with many disciplines) models and modes of analysis that reduce complex phenomena into manageable units. The problem is when economists assume that their models are the reality that they stand in for. At a certain point anthropology is about exploring what humans actually do based on close and fine-grained attention to the tiny details that fit together to form the incredibly complex social, cultural, political, and economic realities of our lives.

The map is not the territory that it seeks to describe, and if your model must assume that Christianity and Islam are two ontologically distinct entities (instead of complex systems of culture that move around the world with those whose culture they are a part of) then your model might be useful, but it's like a earth-centered view of the solar system: it's totally useful for predicting many commonly observed phenomenon, but it's totally and absolutely wrong and ends up being a barrier to understanding the subject that you are ostensibly attempting to understand. If you are really seeking to understand Da'ish and political ethnonational religious-based violence, then you need to at least recognize the real complexity of what it is you're studying. The role of anthropology is to use models if and when they have explanatory force, but to always remember that they are not and can never be anything more than a tool to make sense of real human complexity.

Look, the idea of understanding Da'ish is great, but if we just accepted your premise then what's the point of doing anything about Da'ish or religious violence? Since it will eventually evolve away, we are left with a much more bleak political reality where we are left with no legitimate way of counteracting violence. I mean, come on, if your prediction holds water then what policy options or help does your model serve? However, a genuine understanding of a cultural system does allow you to both understand (and in the case of Da'ish, intervene) that system in ways that can actually be used to stop, curtail, or reduce the violence that is destroying heritage sites, focusing violence on women, members of alternate religious communities, etc. etc. etc.