> > Monopolies are predominantly, in anti-trust law, judged by market (pricing) power, not raw market percentage.
> In theory sure. In practice, explain the Whole Foods/Wild Oats case
Pricing power explains that. Pricing power occurs when price increases from one market player do not result in losing business to a competitor; the fact that other players could in principle choose to enter the market and unbermine that pricing power is not an argument against the existence of pricing power.
> Trader Joe's/Albertson's/etc could start carrying the goods overnight.
But, whatever one might argue they could do, they didn't in places Wild Oats or Whole Foods were alone in the segment, with price competition occurring where one of the two opened stores where the other already operated, and avoiding that price competition being documented as Whole Foods own analysis of and CEO’s motivation for the deal.
> My point is that there can be "effective competition" even if there are zero competitors at the moment
Which is false. There can be speculative, potential future competition, but that's not what anti-trust law cares about, for good reason.