While we do offer bonuses for upvoting our posts, there is no obligation. Every component in our system is 100% voluntary and people choose to participate or choose not to participate based on their own evaluation.
We have done a whole series in the past on how our sustainability metrics work and the efforts we undergo to ensure that Steem Basic Income never becomes a ponzi scheme, even accidentally.
Steem is fixated on a content-based curation model -> what is the value of this piece of content? Should this content have been rewarded so high? does it have paid votes, etc.
We are building a patreon-like account account-based curation model on top of that. What is the value of this account? Am I willing to contribute to a crowdfunded support model for it?
You started with a reference to the rewards pool as a limited resource... which is true in a sense. However, the value of that limited resource is unlimited. A low-friction patreon-like support model could bring massive amounts of value to Steem, increasing the value of the reward pool for everyone, regardless of whether or not they choose to participate in patreon-like support.
Try to look at the big picture?