There's no mining or staking it's 100% Proof of brain

Actually, you can stake POB and you must do so to earn curation.

very 4 years POB rewards will decrease by 50%. That means if you earned 100 POB on your post and the halving happens you'll now be earning just 50.

You miss out the factor of a growing community. As the community grows, the rewards get spread thinner and thinner. So rather than getting 50 POB, you may get 2 POB for a similar post. It's going to get a lot harder to get POB in four years. The total reward rate given to everyone in aggregate (everyone's reward added up) will be 50% of what the reward rate is today (in sum of what everyone gets). At the beginning this is 50 POB every ten minutes. Four years from now this will be 25, but posting rewards will get harder quickly. Voting power is going down too. I remember when my vote was worth 77 POB and today it is half that. I don't want to be the one to complain on a full belly. It's an observation.