[Lalwendë]

Maybe they thrive after the Rohirrim promise to leave them alone and not hunt them any more?

I suppose that like any old group of tribespeople their survival would depend on a couple of factors:

Whether their environment survived and hence they could maintain their traditional way of life. Many rainforest tribes have suffered from having loggers cut down the forests and have been forced out to live in modern settlements, even forced to live in cities; this is something which has happened a lot in South America. Once the land is gone, the culture dies too.

Whether excessive contact with other people harms their way of life. This is a common factor in destroying old cultures. Unfortunately the explorers and missionaries played an immense part in destroying global cultures throughout history, imposing other cultural ideas upon people and forever destroying ways of life. The last place in the world with undiscovered tribes is Papua/Papua New Guinea/Irian Jaya where some indigenous people still live who have remained untainted by contact with western culture.

Whether economic/social factors would allow them to carry on. Many tribes lose land due to other cultures taking it from them - history's full of this, notably the Native Americans. Every nation has a history of both perpetrating and suffering it seemingly, including Britain.