Well, I for starters read your post. So we can conclude, that some do.

I will try and answer your post with this question. I hope it will make sense to do it this way.

Are Steemit users only in for the currency they can accumulate?

Many probably are, but don't forget that the same is true for every other big social media. If you take a look at Facebook, you will see no end of pages copy-pasting "funny memes" and "fail videos" on the off chance of becoming the next 9GAG type of page, and some day earning some ad-money.

Under each funny post, you will see a "funny" comment by a public user account trying to harvest extra likes. Oftentimes it is barely related to the original post. It will be the most upvoted, even if it is blatantly stolen from somewhere else, simply because the person have been doing this for so long, that he have accumulated likes.

On YouTube people copy-paste videos or upload zero-effort reactions to get some cheap likes. Commentators post the same nonsense on every popular video in hopes of getting likes and sometimes views for their own channel. On social media like these you even see the classic: "If you follow me; I follow you", it provides an insignificant amount of value compared to a follower who like your content, but people do it everywhere anyway.

Some people even try to post upvote-bait on Reddit because they love the virtual recognition they feel.

Facebook and YouTube sure hasn't fallen anywhere. That is because they also provide value to many. Steemit provides value to me reading your post and maybe to you reading my comment or someone else's post.

On a slightly other note, I don't see how you can discount J.K. Rowling, since she has spellbinded a generation of readers and now mainstream popularized the idea of a protagonist growing with "his" audience. Growing with its audience is something Steemit will most likely do as well. It has only been slightly more than a year, so we cannot discount it just yet.

Even if Steemit somehow fails, someone else will pick up the torch, like Facebook after Myspace and Google after Altavista. However, I don't think it will fail only on people just "playing the game for cash", as people do that everywhere. The only difference is that Steemit, at least currently, takes away the middleman and the constant Facebook style overflow of sponsored messages.