It’s rarely about literal monopoly. It’s more often about market power. Choice is one tool that consumers, and small scale producers have to ameliorate some of the consequences of market power.

Of course having market power, is not the same as abusing it to harm consumers - but if the profit margins are ‘abnormal’ and persistent and or uneven throughout the supply chain that’s sometimes a good indicator. If choice/competition isn’t whittling down excess profits in the long run, then there might be some distorted power relationships somewhere in the market or supply chain.

Proving/ disproving the alleged ‘unwritten rule’ might be quite interesting though. But competition investigations need to be quite wide ranging and it is notoriously difficult to prove anything.