While it’s true that the conquistadors had immense success in their initial encounters with the natives, the Aztecs would have adapted eventually to counter the Spanish had they not been crippled by the European diseases. This seems to have already been the case in the siege of Tenochtitlan where the Spanish suffers huge casualties despite having similar numbers (together with their allies) to the Aztecs. This was the case with many of the North American tribes and even with the diseases it took the Spanish another 150 years to subjugate all of the Mayan cities.
I’m not saying that they would have defeated the Spanisn in the long-term but conquering and permanently occupying all the native states in Central America would have significantly more expensive for the Spanish. Without the diseases I think it’s not unlikely the Spanish empire in the new world would have more closely resembled British India i.e. a bunch of semi independent states and directly controlled areas with the majority of population still preserving their culture and language. And in general European colonization would of the new world would have been more similar to what happened in the rest of Asia and Africa.
