The Nahuas are still the Nahuas. There are at least a few million of them today. Most of them live in Mexico, but there are some in El Salvador as well as in emigrant communities such as in California. Many still speak Nahuatl, which has gone through changes since the arrival of the Spanish but is still very much the direct descendant of the language spoken by the Aztecs.

The reason we still call the Nahuas Nahuas but we don't call Indigenous Peruvians the Inca is to do with the different criteria defining Nahua vs Inca. Nahua is a cultural group, one which existed before the Aztecs (though historians debate the exact nature of this) and which continued to exist after them. Inca, like Aztec, is a political group. The Inca spoke Quechua, and there are millions of people today who still speak a Quechuan language. However, we don't call them the Inca just like we don't call the Nahua the Aztecs because that political system was broken down. Our word Inca comes from the name of the Inca leader, or Sapa Inka in Quechua. The Quechua name for the Inca empire was Tawantinsuyu, meaning "four parts together", which referred to the political organisation of their empire.

There are still people today who descend from those of the Inca Empire. They are in Peru but also in Bolivia, Ecuador, Chile, Colombia and Argentina. Just as we call the Nahuas Nahuas based on their language, these people are sometimes referred to as Quechua people based on the fact that they speak Quechua. There are estimated to be between 10 and 11 million Quechuan people today. The social divisions between Indigenous and non-Indigenous Peruvians are very real, so while some Peruvians are Quechuans and some Quechuans are Peruvians, it is not a one-to-one correspondence. Same with Mexico and the Nahua - there are many Mexicans who aren't Nahua, and some Nahua who aren't Mexican, so it's more accurate just to refer to them by their specific name. Hope that clears it up!