So, here's how I've come to think about this stuff with the little I've read, listened to, etc.
Blockchain is like the internet. The technology is likely legit. It's likely useful. It's unlikely to go anywhere anytime soon (though it's certainly possible some alternative could spawn that makes it obsolete, it seems pretty remote).
All these cryptocurrencies are the .coms that sprung up in the late 90s. They're seeing the success of others, they're popping up all over the place, and the money being thrown around is going to ensure that they continue to emerge. Some will have noble intention, utility, and value. Some are going to be scams, pump and dumps, etc. Some are going to suffer from valuations so extreme that no matter what their original intent or value, they could be crushed by the weight of it.
The most likely scenario seems to be a replay of the .com boom. More and more people are going to get into this until the money and the number of new entrants cause the valuations to soar to unsustainable levels, and MOST of these players, especially the newer ones will cease to exist. And the people holding those coins at that point in time are going to get absolutely hammered.
Out of that, though, blockchain is likely to persist, the same way the internet did even after the .com bubble burst. And some of these cyptocurrencies are going to become Google and Cisco and Amazon. The questions are: What stage of the lifecycle are we in (seems early, but not for every individual currency/.com)? Can you pick a few winners? Can you get value out of the losers before they become worthless or find a way to identify and avoid them altogether?