It is important to note that you don't need an inflation-based reward pool to earn with your posts. Literally no other platform economy, let alone any successful ones, pay their users through inflating a currency. We're still early on in an experiment to learn if this is sustainable and can work at all.

While I won't claim to know for sure (although I do believe so), it is very possible that we end up realizing that using inflation and stake creates more harm than good:

• Unsustainable for token value, thus not only harming investors, but users too, as their token earnings also depreciate.
• When rewards depends on stake-based-votes it creates a situation where users are only incentivized to share their content with those who have high SP, since only they can provide valuable votes. This is imo on of the biggest reasons Steem has not grown, as the whole network effect does right there. Literally every other platform gets its strong network effect due to the fact that creators who want to earn has incentives to do the exact opposite: To share their content as far and wide as possible to get more attention.
• It is possible, and I think quite evident, that although distributing rewards through votes can be fun and interesting, it is also really toxic for a social media. People end up getting frustrated with downvotes, and look at the money in votes before they look at whether or not content and interactions felt valuable. Imo it would make more sense to have more traditional voting, and allow reward distribution that is not derived from inflation to happen outside of voting on posts.

But in any case, rewards doesn't have to come from inflation of STEEM. It can be an SMT like Hive account@geekgirl said, or even with liquid STEEM (my preferred option. Ideally, there would be an innate patrean-like model on Steem with liquid tokens, or allow communities to earn STEEM through ad-programs, community-related perks, promoted articles, and 20 other ideas I could mention. And then allow that liquid STEEM to be distributed as a community-specific reward pool where a portion goes to content creators and engager, and perhaps some to promote the community, or add features through a community-specific SPS. The options are many, and the reward pool using inflation is needed for nothing.