How come themes like BIONICLE can afford so many new parts?
Bionicle sets don't actually make as many new pieces as they did. 

Typically, new pieces are weapons or heads, and LEGO company is already cool with green-lighting those. (They're sorta important.)

Gen 1 Bionicles did, admittedly, have a lot of new pieces, but Gen 2 sets, you will frequently find, try to recycle concepts and pieces, just in different colors or with different printing as much as possible.

Hero Factory was pretty revolutionary, with pieces that connected to themselves, (ball and socket,) but for that those were made, you'll see them incredibly often as staples of new sets. That particular kind of new-piece binge shouldn't happen again for a long time, if ever.

I don't know about a dozen pieces in one set though. You'll often find that "new" pieces are just old ones recolored, reprinted, or recycled.