You're right, the only thing that will make Steem successful is tons of quality content. I've always advocated that ideally Steem(it) would focus on getting the best content out there rather than trying to come up with fancy features that only foster greed.

Of course those posts are a double-edged sword, but for sure the rewards they earn won't be enough to have a visible effect on STEEM and SBD prices.

Long-term prices will only remain stable if buying the tokens is attractive to users. Steem hasn't been successful at coming up with the best scheme for that, and it's hard. In any case, an attractive platform in terms of Steemit (and its competitors) will sure increase the likelihood of STEEM being a good investment.