i think the threshold for anti-trust in the US should be when the competition is reduced to the number of people who can fit around a good (but not great) dim sum restaurant's banquet table, so like 8-12. If all of the CEOs of the companies that are within a competitive amount of revenue to each other can fit around the table, boom, anti-trust laws trigger, break 'em up. If you can all fit in one table at a mid-tier dim sum restaurant you're no longer able to be considered in competition because the negatives of collusion basically disappear.
(competitive amount of revenue being like, within one order of magnitude of each other, instead of 'basically insignificant' competition that can't catch up without some black swan happening)
