With enough websites using this API to block "untrusted" devices, since Google is the one that decides who is "trusted" that gives them an iron grip on what you can use to browse the web.
If they decide to not grant your competing project "trusted" status, you now cannot build a competing device, OS, or browser. And since Google owns the most popular browser and one of the most popular OS that gives them little incentive to allow competition.
Without a "trusted" browser that can scrape the web you are also effectively blocked from building a competing search engine.
Basically this API serves to cement Google into a position of power.