Most genetic studies show the average Mexican has around half and half Native American and European ancestry with about 5% African ancestry. 99% of Mexicans speak Spanish and 94% speak only Spanish.
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.
Mexicans are indeed a new people drawn from both native and European stock and a fusion of those cultures. There is a notion of Mexicans (or even Latinos) as la raza cósmica, which is deeply connected with Our Lady of Guadalupe, regarded as "the first mestiza". This mestizo identity is core to Mexican identity. It isn't colonial even if colonialism served as a vector and a catalyst for it.
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.
I was not suggesting "going back" to some sort of medieval past. Aboriginal languages and cultures do exist and they are oppressed. They are not fictional. Oddly, your arguments sound like Putin's points on Ukraine and "fictional" Ukrainian culture. 40 years ago, they all spoke Russian, and the moment they tried to unite around an indigenous, a more deeply connected culture for them, they got attacked by a colonial power.
40 years ago about 2/3 of Ukraine were native speakers of Ukrainian and primarily spoke Ukrainian at home (close to the same number as today).
A little over 1% of Mexicans speak Náhuatl (the most common indigenous language).
There is no comparison here.
If by decolonize you just mean stop oppressing minority cultures and languages then that sounds great. But decolonization is the wrong word for that.