I think it's worth pointing out that, had the entire expedition of Cortez been wiped out (say, during the escape from Tenochtitlan), that's over a thousands Spaniards dead, which would seriously deplete the available manpower in the Caribbean at the time. The Spanish aren't going to be launching another expedition against the Aztecs anytime soon, even if they had wanted to.
I agree with those who say that the Aztecs would have had an extremely difficult time adapted Spanish technology. They simply didn't have the available knowledge, nor was their culture one in which technological came easily.
But , even IOTL, they did show a remarkable ability to adapt to Spanish tactics. Realizing from bitter experience how great an advantage the enemy cavalry gave them, the Aztecs changed tactics and forced the Spanish to engage in house-to-house fighting, where their cavalry was not of much use. They also devised clever tactics for disabling the small ships the Spanish had constructed on the waters around the city.
So, if Cortez and his men are killed and/or sacrified, you will have at least a decade, if not longer, for the Aztecs to absorb the immense cultural shock the invasion would have produced, endure the first and worst wave of the smallpox epidemic, and reassert their authority over the subject tribes that had allied with the Spanish.
Even in the later days of the campaign, the subject peoples remained in terror of the possibility of Aztec vengence. On one particular occasion, when the Spanish suffered a significant tactical defeat (resulting in dozens of Spanish soldiers being sacrificed in full view of their comraces), most of the Indian allies fled the theater of war and could only be cajoled back when it became clear that the Spanish were still in business.