I'm willing to bet that Google will be the first trillion dollar company. Their bench is just too deep. The only major mis-step I've seen is not aggressively rolling out an apple pay competitor 1-2 years ago (Giving away terminals if they had to).
The major threats Google faces right now are: (1) Precision marketing from Facebook (2) Companies like Amazon get so big that users side-step the need for Google discovery. Also an AI business can be as big as their search business (AI as a service in the new smart economy.)
Addressing these: (1) Most people use Google far more each day than they use Facebook so there are just more opportunities to monetize. Frankly if Facebook disappeared it would be a minor inconvenience to most people, whereas if Google disappeared it would be armageddon. Facebook is always at risk of being superseded by another trend, but Google has become a necessary tool. Furthermore, most people check facebook 2-3 times per day and the real estate is far more limited in a facebook feed (Albeit far more valuable). Finally, Google still has Gmail which is basically the world's second biggest social network and it can be mined for far more valuable insight for even greater degrees of precision marketing.
(2) We only have room in our brains for handful of companies like Amazon, and Amazon is primarily limited to consumer goods. That still leaves the rest of the economy that needs discovery: cars, investments, nose jobs, viagra and fake twitter followers. Still, Google does need a mobile/local ad strategy. However, they can pool search history, Gmail, and track your physical location (Oh and they have maps) and start inserting hyper personalized ads/offers into their search result feed. Add a payment system to that layer and they'll crush it.