Like Craigslist and its downfall, the migration isnt going to be to one large community, but many smaller ones.
https://www.quora.com/Why-hasnt-another-product-disrupted-an...
Which had quite a lot of visibility a few years ago.
So stuff like Voat, the shut down Imzy, Hubski and Snapzu represent parts of what Reddit was, and the different aspects of the site that different communities either enjoyed or now want.
What makes you so sure of this? I would argue that this is wildly inaccurate as there will almost certainly be another platform (probably many as you look outward into the future). Imzy was poorly designed, branded and executed (internet safe space?). While their effort and aspirations were righteous, they didn't deliver something people wanted. Honestly I watched that platform since the day it launched and knew it wouldn't last. Digg is old news. I've seen others like topick.com that tanked as they weren't innovative enough. Gab.ai is gaining some traction but it's product isn't innovative and it's too political so it won't scale.
The barrier of entry isn't that high for a new social platform, it's only a matter of time before something newer, less corporate, less "reddit" comes along and gets people's attention. The same could be said for facebook, twitter, and others. No one has a monopoly on ideas.