Basically how christians have always operated since they first emerged and started doing the same throughout the hellenic world. Most (all?) christian traditions are pagan in origin. The main goal was not necessarily replacement but a sort of corporate ownership takeover of the littany of existing traditions, such that these cultures might be ingrained into a unified fold of christian power/influence/customs/laws/expectations and not see themselves as some distinct independent people. And boy that model worked to tame europe and much of the known world under the whip of the pope in a way the roman emperors never could, at least until Martin Luther screwed up a bit of that unity.
Mexico is interesting because it is a certainly a different catholicism than what is practiced elsewhere. Tell a devout old world European catholic about say Day of the Dead traditions and they will probably think its blasphemy. There is a lot of what amounts to local diety worship still as well, although they just label them as some patron saint of somesuch to preserve the veil of monotheism.
> Tell a devout old world European catholic about say Day of the Dead traditions and they will probably think its blasphemy.
Want to make a Catholic mad? Tell them saints are just minor gods of the Christian pantheon.
I don't think Mexico is an exception in that respect. There are some pretty significant Incan elements in the Catholicism practiced in the Andes, Vodou and Santería stuff in the Caribbean, El Tío in Bolivia, Tetri Giorgi/Uastyrdzhi in the Caucasus, and lots of others no doubt. Christianity (and Western culture) didn't demolish these beliefs and practices.
It is not true that Christian practices are mostly pagan in origin. You do not need to look far to find the origin of the vast majority of what you would have seen in a medieval church service. You can see the common origin if you compare and contrast an Orthodox Jewish synagogue service with an Orthodox Christian liturgy. The first half of the Divine Liturgy and the synagogue service are virtually the same even in terms of the motions — the Torah scroll is replaced with the gospel book. Christianity owes far more to second-temple Jewish practices than it does to anything pagan.
It’s worth noting that “monotheism” is a modern word. It was not a word used by theologians of the pre-modern era. The Bible is insistent that God is in some sense “one” or “alone,” and that you don’t offer sacrifices to other gods. People just thought that other gods were demons. It made a big difference to this point of view whether a sailor was asking St. Nicholas to help him get home safely or asking a demon for help, in the same way it makes a big difference whether you call the local mob or the municipal police when you have a problem. The latter are quite analogous to an angel/saint because police don’t have power or make laws in their own right, they are just servants of a higher authority (ultimately, the constitution). The mob, like the demon in the Christian point of view, are in disobedience to that higher authority or in direct competition/rebellion against it. There’s a really big difference even if both paths end with armed men with guns coming to solve my problem.