Their best chance is even before the spanish arrival, when the incas tried to conquer their lands back in c. XV. Fearing a new wave of conquests from their northern neighbors, mapuches could have banded together in city states or something like that with a common culture and intricated trade networks. So when the spanish arrives in c. XVI they find a unified yet descentralized group of many mapuche tribe-states cooperating to prevent inca advance. Lets assume that diseases takes their toll and many mapuches died during the conflict, but key people like Lautaro or Caupolican lived long enough to unify under a single flag the mapuche. This brutal resistance wasn't unnoticed in Europe and soon other colonials powers would like to have a trade deal and cooperate with the mapuche nation just to mess with the spanish.
Fast forward decades and centuries, when Chile got their independence, the lands in their southern frontier were inhabited by way more natives, organized and centralized in a single nation under the protection of a european power that slowly granted them economic and diplomatic independence. Chile wont go south if it means war with the british, french or other empire and would let the mapuche live in peace. Eventually trade relations and cultural interchange would be common. The mapuche nation could be a thriving native american nation after all and even they could be as developed that their chilean or argentinean neighbors.