[monkeydreams]

> 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.