Google does lack a sufficient "moat". If someone were to have a demonstrably better take on finding answers to what people are looking for on the Internet (be it traditional search, or something else), Google might not be able to pivot quickly enough. Competition is only a URL away so switching costs are low for end users. Google's real customers (advertisers) will follow quickly if users leave.
Being a URL away is a pretty big thing. Their moat includes being the default search engine for Chrome and Android devices. They've also made deals with everyone except Microsoft to be the default search engine on every browser/platform/device you can imagine.
