This is often repeated, but historical evidence does not support it.
Today, Google shares the majority of it's presence in online ads market with Facebook.
Did Google try to "kill" facebook? Yes, they tried super hard in 2011 with the launch of Google+. And that completely flopped and now Facebook is a major rival for Google.
If we look at Facebook - facebook probably tried to kill snapchat and tiktok, major rivals to Instagram, and look how well those services are doing.
Microsoft couldn't stop iOS or Android.
The list goes on...
I think a competitor could rise by doing the same thing tech giants do. Start small in a unrelated niche than with the success in that niche enter other sectors until you can compete with Google.
If some other innovative company “started in a garage” or whatever does challenge one of those, Google has the power to kill it before it becomes a threat, cripple it while they copy the technology or poach the development team, acquire the company with a truckload of money, etc.
So in the end, Google will always come out on top, and consumers will always be stuck with them. The only remedy to that situation is government intervention.