> I think most people can accept that there's a risk to consumers, markets, and democracy if companies become too big.
I don't think that's true at all. If you want to make an argument that companies are too big, you need some exact logic to support it.
The only solid arguments I'm aware of are specifically regarding banks because of their systemic impact on the economy -- the become "too big to fail" and thereby become a moral hazard situation. Although given the efficiencies of large banks, the solution has become to regulate them more tightly to prevent moral hazards, not to break them up.
But the idea that tech companies are too big doesn't have the same kind of logic behind it, and your assertion that they "snuff out entire startup sectors" doesn't seem to be supported by any evidence. To the contrary, they invest in entire startup sectors and competing top tech companies buy competing startups to supercharge them. Competition is thriving as the big tech firms compete with each other.
In the modern era of Big Tech, consumers seem to be doing great, markets seem to be doing great, and Big Tech's size is probably not even in the top 50 threats to democracy. The effects of social media is surely in the top 5 threats to democracy, but that has nothing whatsoever to do with the size of the company that owns a social network.