Try telling people who have only Comcast to not get off the Internet, or people with only one nearby grocery store to boycott it.
Both require strenuous oversight, and without a govt capable and willing to constrain that oversight, you wind up wint an economy like the 1890s robber barons, which needed to be broken up by Teddy Roosevelt
Regardless of the personal cost to not doing business with them, they won't send an armed force after you that will kidnap you. But with the government, if you want to grow a plant on your own property that they don't agree with, they will. That makes a massive difference.
But, with copyright and other corporate pet issues, is it the Govt that really cares about sending the armed people to arrest and charge you, or the corporations that effectively own the govt?
The further that excess corporate power is allowed to spread, the more tyrannical it will become.
My generalization is that for a society to function, while some concentrations of power are necessary (at a minimum to allow specialization, e.g., infrastructure, policing, military, legislation, executive functions, etc.), the key is to minimize and actively avoid concentrations of power.
The largest and most dangerous concentration of power in modern times is the corporations, as we are seeing with Cicizen's United, decision, regulatory capture (FCC, Copyright law extensions, current dismantling of EPA, other regulatory agencies, etc.).