Candy Crush may be a bad example... King has been around for 10 years, so not really a startup. They're somewhat similar to Rovio in having a late hit.
It depends what they do with the success that determines their future, I'm pretty sure they're aware that Candy Crush is a fad, just a matter of exploiting it while working on the next thing.
They had an original idea, a great product, they kept releasing new additions to the product. They built a large business around it. They branded the product, they created merchandise, they never scummed their employees and took their early equity.
Don't you think Angry Birds was rather "inspired" by Crush the Castle[1]?
They did well milking the idea for every penny, and I think they were a key driver of the casual gaming genre (along with Zynga and their Facebook gaming) but I think they were pretty lucky with a few key celebrity endorsements.
It depends what they do with the success that determines their future, I'm pretty sure they're aware that Candy Crush is a fad, just a matter of exploiting it while working on the next thing.