> How are you defining "market power in any market?"
This is already a concept that exists under established antitrust law. It's complicated and ugly and could probably use some reform, but it's also a different part of the equation. "Does this company have market power" is a separate question from what do we do if they do, for which the proposal is to prohibit vertical integration.
> Is it just if a company gets 20% of a market? 50%? 70%? 90%?
Market power has very little to do with what percentage of the market the company holds. For example, in a market with two local ISPs where one has 95% of the market and the other has 5%, they could both have market power because the market is so consolidated that the company with 5% could still be able to dictate terms to customers. On the other hand, a company with 99% market share might not have market power, if barriers to entry are low and any attempt to raise prices would cause new competitors to enter the market, as is the case with e.g. Walmart.
> Mailchimp arguably has "market power" in email marketing (70%). Should they have been allowed to get into the social post scheduling business? Using your argument, you could call that unfair competition for social media management Saas tools like Buffer, since Mailchimp already holds market power over one area of the marketing stack.
I don't see the trouble here. Mailchimp may or may not have market power (I don't know enough about that specific market to evaluate it), but knowing 70% isn't really that informative. If they do have market power then preventing them from leveraging it to destroy Buffer is good. If they don't then they wouldn't be prevented from entering the other market.