I agree with everything you said.
For reasons that Professor Iacobucci articulated in his opening remarks, market definition is a very messy enterprise. Reasonable people can disagree about how to define the market, and once you've done that, you're going to get different answers about market share and concentration.
If you have a merger, what you should care about, as you have suggested, is the impact of that merger. How will the market be after the merger compared to what it would have been absent the merger? That information only comes from studying the merger and studying its competitive implications. Some hard-and-fast rule that says you can't merge when you get to a certain size and a certain market share runs the risk of blocking mergers that could be very efficient.
You have mentioned small geographic markets. That is a concern. Sometimes when we use the term “natural monopoly” our minds tend to go to great big firms that dominate their industries. In the old days, we used to think about the electric companies and the only telecommunications company we had. We thought of them as natural monopolies. However, in fact, there are many natural monopolies in small markets. The market is just not big enough to support more players. You might start with very large market shares, so you should allow the bureau and the tribunal, using discretion and giving the tribunal the authority to make decisions on this rather than ordering it to issue an order—