The pre-columbian civilization was far less barbarous. Before, we had an actual waste management system. Afterwards we inherited the european practice of dumping our shit out of our windows. This caused an explosion of dystenery, killing countless every year. The former capital of science, mathematics, philosophy and art was turned into just another massive resource extraction slave labor colony. The effects of that and the caste system damned hundreds of millions to poverty and caused hundreds of rebellions, caste wars, revolts. The biggest of which being the Revolution of 1910 which was the bloodiest war in the Americas ever. What's a couple thousand dead to ritual every year compared to the millions dead due to famine, poverty, slavery, persecution, war and disease. There is no solid "humanist" argument for the conquest. It objectively caused more human suffering than if the empire stayed.
Not to mention the effects of the church. Just a simple crash course: Catholic Hardliners loyal to the church were encouraged to centralize the state under Santa Ana, the state was unable to effectively exert it's authority due to the extremely hard to navigate nature of Mexico. With a giant mountain range splitting the country and no navigable rivers. This culminated in many rebellions, the independence of Texas and the Mexican-American war.
The Church encouraged people to openly rebel from the government after the liberal constitution of 1857 was passed which limited the power of the church(removed it's status as state religion, established secular education and forced the sale of all church property), introduced individual rights such as freedom of speech, assembly, universal male suffrage, federalism etc. It was Mexico's bourgeoisie constitution which solidified the country's transition from feudalism to capitalism. This led to a bloody 3 year long civil war which destroyed the country's infrastructure and sucked the reserves dry.
The conservatives came back a couple years later with the French as their allies. Hoping to establish a catholic monarchy puppet state. This war was of course blessed by the pope. It caused another 5 year long war.
Years later, the serf-like status that the indigenous people had to endure culminated in the 10 year long revolution that gripped all aspects of Mexican society. 2.7 million people died, in a country that had a population of 15 million at the time. It was the boiling point of over a century of resentment. The secular, socialist government that emerged propelled the country into a massive economic boom, it nationalized the industries which were stolen by foreigners and instituted sweeping land reforms which increased quality of life. It further limited the rights of the church, in response the church rallied for another civil war. Finally emerging from this last war, the church was never the same. Some of which still survive to this day
-All churches built before 1992 are considered national patrimony and are fully owned by the state
-All churches built after 1992 are owned by their respective religious organization, but are subject to taxation
-No religious ceremonies are allowed outside places of worship without a government issued permit for that event. Religious ceremonies conducted outside of places of worship are subject to regulatory law
-Members of the clergy forfeit their right to participate in government, advocate for partisan political views, support political candidates and cannot publicly oppose the laws or institutions of the state
-Religious associations may not hold political meetings of any kind or own or operate radio or television stations
-Government permission is required for commercial television or radio to transmit religious programming
-Socialist education until 1930s, when it was replaced by Secular education
Before, from 1930's to 1992, the laws were much harsher. In that time the church's influence waned dramatically. Nowadays, there can be more leniency since there is almost 0 church influence and therefore no risk of church interference.
What's funny is that from this I can draw a parallel between what you said about the old gods instantly collapsing and "the lord" instantly collapsing once independence was achieved. "Just pray harder bro!!! Im sure jesus will intervene right before you're about to be martyred"
Also humorous is the distinction you place between the old and new faith. The lines are extremely blurred since the spanish themselves didn't care how heretical we were as long as there was a facade of faith to keep the empire stable. The mass conversion of the Mexicans only happened after a brown marian apparition in the same hill where the former temple to Tonantzin (Our Lady), afterwards they just accepted her as an aspect of the former deity known as Tonantzin Coatlaxopeuh (Our Lady who emerges from the light like the Eagle from fire). If you look beyond the surface level, Mexicans never completely got rid of their former faith. They just dressed it up with abrahamic elements.
