My concept of it was that it would force people to keep coming back if that were the case, even if it was just weekly's:
Player logs in to do weekly challenge, but hasn't played due to lack of motivation, sees a cool update and wants to start playing again.
The reward doesn't have to be skilling related, although that's the point OP made. There could be an overall challenge to complete X amount of daily challenges, once that threshhold is reached, you'd be able to unlock certain cosmetic items (as an idea). And to ensure that people stick to doing these challenges, make the cosmetics discontinued monthly and change them. (Sort of like how fortnite seasons work where you're unable to unlock outfits from previous seasons, so you look more of a veteran for having the oldest outfits).
Cosmetics is just one idea of the plenty that the community could bring forward.
Spoiler for Example:
Getting back on topic: My suggestion was not meant to change the subject at hand, I just wanted to throw the idea out there and if it gets enough support I'll make an actual thread for it. But let's get back to talking about OP's idea.