The idea is that with a 5$ HIVE for example, 100HIVE as a weekly reward might be too much to giveaway for a post.
That's the reason for the change to HBD at some point, we may fix it to say 50HBD, 25 for first, 15 for second and 10 for third place, that sounds fair.
Yes, I think it's fair. Stable prize means you just converted to HBD and sent out the rewards which a reward amount is already set.