reward pool goes up and down based on traded price of steem. as long as their is confidence that steem has value and is likely to be tradeable to BTC or USD.

as long as confidence continues and grows, price of steem grows, and the reward pools grows.

two different perspectives of this situation

one view is this is a Ponzi scheme, the value of steem and reward pool will collapse to zero once the Ponzi scheme pulls in maximum users. early in gets most of the rewards.

alternative view is there is not that much difference between steem and USD. Once confidence in USD collapses, the USD will collapse and be worth a lot less. just like a Ponzi scheme. the lasst in to hold USD will get wiped out when the Federal Reserve Ponzi scheme collapses