With anti-trust, it’s defining monopolies. Maybe a company like Google dominates search advertising. But they don’t compete economically in search, they compete economically in digital advertising - or is it all of advertising? Isn’t Amazon and Apple eating their shirt in new spend on search ads? By the time the anti-trust case works itself through the courts it’s often that the “monopoly” has fallen quite a bit from their perch.
Now to get to start ups. Startups being acquired WANT to be acquired - otherwise they wouldn’t be sold. There is no gun that makes them be sold - their shareholders agree to it. If you love startups, you should be pro-founder, and that means letting them be sold if they want it. Otherwise it’s simply coercion and authoritarianism to prevent an owner to do what they want with their company they put their blood and sweat into.