Seeing it as a business, it wouldn't be a smart move considering that they commercialize the containers and TB tags that would decline as well if there weren't containers to hold them between cachers.
They can, and probably will, do both. I see scenarios where a cache is not a possibility (inside museums, above a desert dune, under water, etc.), and challenges may work there. The pirate in me loves the thrill of the find and the expectation while I open a cache, even if I am the TNLN type. That's where the beach grotto and the trail side containers will continue to excel, because most of the community seems to like those. Even the cleverly hidden nano in plain sight to thousands of muggles is exciting in its own way, more than simply going to the place with my gps and snapping a photo standing there to check a challenge.