The complaint is that the offer is a great deal with no downsides for consumers, and this is likely to result in Cloudflare having a lot of power (which they currently don't have) as a market maker. This position as market maker would grant them the power to extract economic rent from the web economy by charging both sides of the web provider and web consumer market to get access to the other.

So the complaint is that one day they have such a strong monopoly that they can freely turn evil?

Just want to make sure I understand the real issue here, because that sounds like a lot of fearmongering to me.

Uhhh, they have it right now, and they are currently being evil by blocking a lot of people from accessing a lot of websites.