Is Google Search going to be the dominant way to grep for information on the web forever? Magic 8-ball says answer is unclear. That's Google's biggest revenue generator.

I'd rephrase a bit...

Remember that they are an advertising firm, not a search firm.

Their ad revenue depends on two main factors: volume of impressions, and the quality of their profiling data.
Obviously if their search market share were to fall, it would hit impressions in that segment, which is of course the largest segment. It wouldn't affect the other sources of impressions, though: Youtube, third party sites, Maps and so on. So the drop in overall impressions would be smaller than the search market share drop. They also have a few other revenue sources that would not be directly affected, notably the Google Play store with revenues in the neighbourhood of $50 billion at an operating margin around 65%.

I don't think it would affect the quality of their profiling data all that much. They still have lots of raw material for profiling, and it comes from a lot more data sources than most people realize. All the content of all messages to or from Gmail accounts, "login with Google" sites, Youtube history, and so on.

So if their profit is primarily the product of two numbers, I think one would drop a smaller amount than the search share, and the other would not be affected, so the hit to the product of the two factors would not be all that huge. Unless the search share dropped by half or some such thing, which is certainly possible but seems very unlikely.

Any strong business can weaken, but I don't see a clear and present danger.

Jim