It does not make a significant point because the actual numbers have already shown the marriage numbers are dropping, the rates are just in keeping with those numbers.

It is irrelevant who is NOT getting married like baby boomers, as there are also people dying and being born everyday, in addition to people planning to marry. Not to mention new migrants/citizens.
Hence, it just gives the rate out of the population hence, over just one thousand.
It is done in most other data calculations like births, crimes, employment, nothing to do with young or old people in the population. Just merely to reflect the rate over a 10 years or so.