It sounds like you are saying that Steemit and DTube can still be big successful things without replacing those things that are similar but different to it. Facebook started out as similar to many other communities, so did YouTube compared to other video sharing communities. They were not radically different. Twitter was just another micro-blogging platform with a silly 140-character limit.
However, lets not forget that Steemit is still centralized - someone is running those web servers somewhere. There could be other frontends, but there aren't. Also, we haven't got even close to the user base of the leas popular online communities and an awful lot of people seemed to be really afraid of what would happen to it if we did have many orders of magnitude more daily users like the big guys. There's no guarantee at all that it would hang together or be something awesome that blossoms to global appeal. We could easily drown in a sea of spam and be crushed underfoot by robots like those skulls in The Terminator.
However I agree that even if we can build a better social media-like site with blockchain these are not the most interesting things to do with it, and probably the most interesting are as different as the blockchain itself from anything before it.
That said there are still scads of things that we use today that have completely flawed implementations and yet are still popular. They could be so much better with the blockchain. It is not a mistake to chase after, for instance, the perfect open naming or identity system, the perfect open asset tracking system, payment services, DAOs, DAXs, etc. etc. As a real-world analogy remember, for example, the SSD drive is very similar to hard drives in concept but hugely disruptive because they are so small, reliable, and stonking fast.