The cryptocurrency projects that will survive and thrive will be those that let people do things that are insane, and yet they want to do anyway. It'll be new markets where currently no economic activity is taking place, because the participants cannot trust each other (or make use of the legal system to trust each other) enough for any rational actor to consider transacting. Cryptocurrencies, of course, alter that rational calculus by letting you put trust in an anonymous network of worldwide miners to secure your transactions.
Think of some of the biggest companies created in Web 2.0. Facebook - who in their right mind would give Mark Zuckerburg all of their personal data so they could hook up with that hot girl in the dorm across campus? AirBnB - people actually invite strangers into their homes to stay with them? Even the founder and first investor call that "The worst idea that actually worked." Uber - you're going to get in a car with a complete stranger and pay them to drive you places?
(Interestingly, all of these are markets that are well-positioned to be disrupted by cryptocurrencies. The key elements of an EBay/AirBnB/Uber-type marketplace are 1.) ability to connect latent demand for a service with people who can supply that demand 2.) ability to receive payment and 3.) a reputation system to ensure that the service was performed reliably. Right now, the UI & scalability properties of crypto are not good enough to let them replace these centralized marketplaces, but all of the information involved could easily be stored on a blockchain rather than a database, and with current Ethereum transaction costs at 0.07c and most of these tech companies taking a 20% cut, at some point the economic incentives to replicate them will become huge.)
We've actually had a couple of these killer apps for cryptocurrencies - buying drugs off the Internet, and ICOs. In both cases, any normal, rational person looking at the behavior is going to be like "What? Are you crazy?" In both cases, people do it anyway, presumably because they really really want to buy drugs without having to meet in person, and because they really really want to invest in startups but otherwise can't.
If you can service a market with databases, digital signatures, and old fashion contracts, you should service it that way. Blockchain is useless for anything that people are doing now, because if they're doing it now, there's already a way to accomplish it.
