Seems to me like you're asking the wrong question with "How can we ensure that the rewards pool is not drained?". Sorry, this might be semantics, but the purpose of the rewards pool is to be "drained" in the first place. By that I mean that the entire pool of rewards is meant to be distributed, with the only difference being the proportion of rewards going to any user.

The question should instead be: "How can we ensure that we maintain the (fiat) monetary value of any single good post over time, despite more users joining and posting good posts?"

Solution 1: I read your very informative post on calculating vote value, and if we increase reward_balance with time, we can help to maintain the vote value. The white paper stipulates a yearly inflation rate of 9.5%, so that will help slightly, as long as the increase in supply is met by an increase in demand.

Solution 2: Except the above, it seems like the only solution is for the value of Steem to increase. As long as the monetary value of Steem increases to counteract the fall in my share of the rewards pool, the overall value to me can remain unchanged. There is no other solution unless we change the formula through which payouts to each user are calculated. At the current rate of increase in users, the value of Steem should increase as well, because the increase in supply of reward_balance is smaller than the number of users.

The problem for new users, though, is that reward payouts are not calculated after 3 decimal places, and any payout smaller than 0.001 is not paid out. This is a problem that actual new users face. That is something I don't understand at all - it is easy to increase the precision of payouts, it'll just at a small cost of making the numbers a bit harder to interpret. If we rely only on an increase in Steem value to maintain the monetary value of contributions to the platform, then this problem will only worsen and become a natural upper limit on how many new users join and stay on the platform.