It's a lesson I learned before the web, when I was in physical store retail - never to depend on one revenue source. It's more of a business lesson than a Google lesson or a search lesson, but it is a survival lesson.
I've told these stories before, but one business was right on the city bus line and had a bus stop in front of the store for 20 years. We could do no wrong and the cash just flowed... until the city moved the bus route to a different street. Top management never got back up to speed because their entire business plan depended on [abusing] free traffic rather than paying attention to all the factors that make a strong business - customer retention, real service, value, market segments - all that.
Another business made 60% of its revenue from selling Levis jeans - until skateboard jeans ate a major whole in the Levis market. They tried to diversify after the fact, but it was too little, too late. They couldn't develop new markets quickly enough.
And so it is with search traffic as a business model. Even when a site is in hog heaven with Google, it is always best to plan for drastic changes. If the bottom does fall out of search for you, then you will be ready.
But if you haven't planned diverse revenue streams, you may well be wiped out. Because its true - no search engine will consider their effect on one particular business when they change things. They will probably care less than the city did when they moved the bus route.
The principle is this: don't have a single point of failure if you want long term success.