Thank you for your feedback (and sorry for the late reply).
Yes, fewer HBDs could be printed but we really don't think this challenge will have such a major impact on the HIVE economy.

Is it a challenge to assess people over a month and reward them based on their monthly Hive power up activity?

It's only for fun and to incentivize people to have a bit more skin in the game with a regular commitment.
Everyone is free to participate or not.