I think kids today have more exposure to downer endings than we had.

Actually the major plot points of Catching Fire and Mockingjay were already premeditated before Collins wrote the first book and she submitted an outline of the series along with her first draft to her publisher. She just wrote the first book a bit more self-contained than the rest in case sales didn't warrant a…

I think the whole "going back into the arena" stuff was premeditated from the start before she wrote the first book.