I think we all need to remember that none of the search engines owe any of us anything.
The argumement that they would not exist without us is not true. The internet existed without most of us for many years between universities and goverment resources.
My point is, I tell my wife every month that we could lose it all at any time. It is the nature of the enormous market we all battle in. If I wanted more stability I would have kept my day job. :-)
This is why i have been preaching for years to diversify traffic. We still are not there, but I work every week to build up my newsletter subscribers, my FB fans, my followers, and i keep in touch with high profile inbound link sources.
It is a cut throat business being inline. Overnight a competitor can scrape you and launch the next day and you may never even know it until they are out ranking you a year later.
I for one will take whatever free traffic all search engines want to send me. No complaints. Free is free.