Who gets to define the market? Does the "market" consist of only advertising technology platforms, or everything involved in online advertising, or all advertising through every possible channel worldwide?

Because bad behavior tends to scale with company size, not because smaller companies are more virtuous, but because they just have less power to do harm than bigger companies.

Imagine that there were only two companies, and they each had 50% market share in every market that exists. Would you expect either to be well behaved?