More uses cases needed, agreed. But there's no problem with a focus on quality. That's not really the reason this platform isn't worth more.
In the first place, there hasn't been any focus on quality in general. And most people who say they want quality don't really mean it. Clearly. Statistically little interesting long form content.
That being the case, the reasons why steem has little mass appeal are the same so far as before.
First of all sign ups are not instant for the majority of casual users who don't know anybody here nor fit into some of the communities that subsidize instant sign ups.
Second, reputation is broken, basically.
Third, people come in expecting to earn; whether they do or don't has little relation to what they post. Primarily because search can be improved. (Efforts have been made in this direction.)
But the largest reason is that there are not billions of resources behind steem, unlike youtube. Marshall McLuhan was asked, Are bestsellers something that happens, or are they more often than not created? He said: They are created. He's correct, I suggest.
Paypal began eating market share by paying users to sign up. Youtube still pays those users it wants to subsidize and doesn't demonetize. Meanwhile the other sites have celebrities on them, and the algorithms encourage users to follow celebrities. So celebrities use them; it's just a coincidence.
More use cases like video posting and games and such will make the platform more successful, but it will have little to do with mainstream appeal itself. It will have more to do with finding a roof and mainstream appeal.
The greatest issue is that users are not the customers of those platforms. Steem depends on value from users. But most users have no money. Almost all investment capital is in the hands of institutional investors and businesses, not mainstream users. Who are the product, as Carl Hewitt recently said. Those other site sell user data, they don't get value from the mainstream use case. Steem does; and then the token isn't worth enough by that alone to subsidize mainstream users who come for the cash. (I don't come for the cash, but I'm not a representative user, for example.)