In that scenario, Google doesn't take pizza advertisement contracts anymore. Want to advertise you pizza place on google? Fuck you, become a Google Pizza instead. Want to sell pizza, that's your passion? It's easy, you just need to create your own search engine on the internet and dethrone google, then you can sell pizza.
People can use other engines, but they don't. If they did, it wouldn't be a factual monopoly. If google isn't put in its place, it can do whatever it wants. They can become a monopoly in whatever sector they want. All in the name of free market.