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.