I share a lot of your optimism. I hope you are right :)

a POS mining platform

Wouldn't a POS "mining" platform that encouraged users to "mine" via curation be beneficial in a similar way?

It is definitely a different animal than the "passive" model that is currently being used, but the issue with the "passive" model is that stakeholders are just paying themselves with inflation. If the inflation is not going towards anything that is adding value to the platform, then over the long term - it is going to cause the price to decrease.

If downvoting becomes prevalent to the point that you have no idea what/if you can earn on here or not, people won't tie up money in SP, why should they?

I seriously hope that HF21 does not result in that. It is obviously very much unknown, but at least from my perspective - the hope is that the majority of downvotes would be getting directed at things that shouldn't have been getting rewards in the first place. If stakeholders are finding quality content to upvote, they should be reasonably confident that they could earn from it.

Basically, I think it will transition some of the rewards from those with stake to those with none, which likely is a further drag on prices as those who get rewards for free are likely the first to sell.

Maybe so, although the more important question to ask (IMO) is whether the users who are getting the tokens are adding value to the platform.

then using SMTs and communities to start rewarding more of the "proof of brain" content.

SMTs (if they ever happen) will have the same inherent problem as STEEM in that there will need to be some reason to buy the tokens in order for them to be worth something.

I am a strong proponent of making STEEM actually work first, and then letting SMTs replicate their success. If the only real demand for STEEM is to use it to direct a portion of STEEM inflation to yourself, then we don't have anything to increase STEEM's value over time.

I also am not holding out much hope for SMTs to happen either. I'm not as pessimistic as many (who have completely abandoned any hope), but I am certainly not going to wait around for SMTs to save us either.

Hope this helps. :)

Indeed. Really appreciate the dialog :)