> It's a little strange to use the phrases "tend to" and "iron law" in the same sentence.
Not really. Tendency denotes a general inclination, such as objects tending to move toward the centre of mass. Tectonic plates tend to shift slowly. In both cases these are very solid and reliable predictions which are almost certainly true in a vast majority of cases.
I would not agree with the statement vis-a-vis luxury items and necessity (fur coats, diamond rings, harems, are not demanded by all), but there is a tendency for certain classes of items to be considered "essential" over time.
May be strange at a first glance but binary/boolean logic/reasoning/quantification is just a special case of probability theory. This is covered exhaustively in Probability Theory: The Logic Science, by E.T Jaynes.
And so "tend" and "iron law" are interpreted as assertions of different levels of certainty, where "tend" implies a certainty greater than 50% and "iron law" a certainty closer to 100%.
A tendency can be shown to be present in all known categories of samples (justifying 'Iron') , while not occuring in every single sample, making it a tendency.
Separately, tendency can mean a type of outcome is more likely, or that there is an underlying force acting in a direction, regardless of whether that force is cancelled by other forces.