There basically is no replacement for Google traffic. Paid ads are not the same since they cut into margin. Plus you can't even buy many brand related terms too.
I think the point is that if something else took over as the king search engine, they'd likely be able to get traffic there too.
Overtaking Google is unlikely, but Google deciding a certain coupon site is spam/scammy and deranking them is quite the threat and would basically kill them overnight.
What is more likely, Google going away or Google changing their rankings in a way that makes you drop 5 positions? That's the risk of depending on Google, imho.