I’d love to know what Spanish decolonization in such a place looks like.
There is no objectively correct demographic language or culture for a given location. You have to pick a point in time to go back to and there is no way to do that that isn’t arbitrary.
The idea of "going back" to some kind of pre-Spanish Mexico is nonsensical, and it would entail the very negation of Mexican identity and the invention of a fictional identity. Such "decolonization" movements are ahistorical. And frankly, I doubt most Mexicans would want a "return", whatever that even means.
Of course, this is different from learning Náhuatl. And it's worth noting that the Jesuits worked to preserve the native languages of the New World. You see this with Náhuatl. You see this in Paraguay where the Jesuits immediately began codifying and preserving Guarani in their missions, and where it is still widely spoken today.