Well, you've got the whole think backwards; the Mayan descendants are doing just fine on the Yucatan Peninsula and other places, while the Aztecs have vanished without a trace (there are no Aztecs blood line anywhere to be found, although a whole bunch of Mexican revisionists wants you to believe otherwise!).
Remember that Aztlan Nation crap they want you to believe it existed and a large portion of the USA's South-West States belonged to that fantasy Mexican ShangriLa?.
Well, how do you explain the fact that there are still more than 1.5 million people in central Mexico speaking Aztec (Nahuatl) language?
Mexico City - the oldest and largest city in America (established in 1325 as Tenochtitlan) never lost its demographic continuity. Well into last century there were neighborhoods which did not use Spanish language.
Actually you are wrong about the Aztecs. The use of Aztec by some Mexicans is like people in American saying "Native American". It is also the case that some of them know that there is Indian blood in their lineage, but they don't know which tribe, so Aztec is used. As for real Aztecs being completely gone, that is inaccurate as well. I know one.
The Atzlan myth was brought into the picture during the Chicano Convention in the '60s when a poet composed and read a piece to invigorate their campaign for rights. He didn't mean for them to bastardize the meaning into what MECHa and La Raza have made it. Beyond that, Montezuma told Cortez in the 1500s that Atzlan was a myth.