A correspondent writes from Denmark to point out this usage seen in The Economist at the beginning of April:
But what makes the rich's behaviour so galling for many critics is that their two greatest crimes were committed in broad daylight...
He observes: according to A Comprehensive Grammar of the English Language (7.23), 'Adjectives as noun-phrase heads, unlike nouns, do not inflect for number or for the genitive case...' And he wonders what is going on.
A change does seem to be taking place among some of these 'de-adjectival class nouns'. A quick Google search brought to light several other examples of the rich's, such as:
At last, a budget where the super-rich's bluff is called.
Close study debunks myth of the rich's tax burdens
And many more examples of other items, such as:
Private education? The poor's best chance.
This would drastically reduce the wealthy's taxes while forcing them to take up more of the slack
Why is it happening? Well, from one point of view, there's nothing new here. Historically, adjectival nouns have taken a genitive for centuries. Plymouth's first workhouse was known as the 'Hospital of the Poor's Portion', which dated from around 1630. And the OED has examples from around 1400 going right through into the 20th century. Both singular poor's and plural poors' are found. The switch to a solely non-genitive usage seems to have emerged in the 18th century. That's when we find poor tax alongside poor's tax, poor money alongside poor's money, and so on. Scotland seemed to keep the 's forms longer. The OED now says that the 's is 'archaic and rare'. It has no examples of the rich's, but that will need to change. The Middle English Dictionary, for example, has an instance from 1425.
Looking at the examples, I can see some reasons motivating the change. There's a succinctness (and possibly greater clarity) in The poor's best chance compared with The best chance of the poor. And this is likely to be reinforced whenever the item turns up within an of-postmodification, as in:
Close study debunks myth of the rich's tax burdens.
The alternative, with its sequence of of's, is not likely to appeal, especially in headlines, where space is at a premium:
Close study debunks myth of the tax burdens of the rich.
The genitive usage may well have originated as a journalistically-motivated change, but it's wider than that now. Certainly worth keeping an eye on.