As L. Frank Baum wrote in the "Marvelous Land of Oz", "Everything has to come to an end, sometime", everything else in life, from hunter-gatherer societies to the United Kingdom being in the European Union, and what we know now as Steem will be forgotten eventually.
Currently, this cryptocurrency might have grown at a fast pace in the previous months, but what about the future? Surely, it will stabilize at some point, with mostly everything about it covered in detail. Despite this sounding positive, I believe it's not. Just like what happened with the Roman Empire at its height, when it was a prosperous and sea-spanning empire: having failed to adapt to the situation at hand, and relying on the principles that were set in stone at its very beginning, it was slowly lost, not being capable of change.
I think that a good solution is giving users the freedom to experiment. As we saw with Reddit, allowing them to create their own subreddits and moderate them of their own accord is a great idea: each individual section of the site has its height, grows, and then one of 2 things happen: either it dies down slowly, or very fast due to external scandals, such as the public's knowledge of pedophilia, or internal ones, such as bad moderation. As is human nature, they will then move onto greener pastures, joining new subreddits or creating their very own, making sure that the users are kept in the site. However, just like everything else, Reddit will also cease to exist at some point, regardless of their ideas.
The question then becomes figuring out Steem's issues and how they could lead to its end. Of course they aren't very clear right now and we have a lot to look forward to, but I promise you that they will be there, no matter how much convincing zealots might put you through, as there is no single thing on this planet that is perfect. Whatever they are, we will slowly unveil them as we go through this experiment, and find a few solutions. As we reach the end, though, we will see them very clearly and then we'll know what went wrong and be able to improve upon it and create something that deals with those problems and improves upon them, just like Steem based itself off Bitcoin, not just outright copying, but adding value.
Finally, of course that Steem may last as long as the Human Race, and it might solve all its problems. However, unless we meet an alien race to share it with, I don't think there is a way to make it live longer, and even with them, what can we do against the impending heat death of the Universe?