Has religiosity actually decreased?
This is fair* (and I expect many Christians would agree with you in generalities of "everyone worships something"); what I was getting at here was that (by the first numbers I found) those who regularly attend organized "god-based" worship has dropped by ~1/3 over the past 20 years or so, and so I would expect roughly a corresponding drop in attendance in parochial schools. If the drop is less than that,** that would seem to support the author's hypothesis.
What were you hoping to add by mentioning this?
Along the same lines as the previous: even if more families aren't able to send their kids to private schools, but desire to do this, the could support two parts of his narrative: (1) decrease in social capital (of which he defines part as, to paraphrase, schools that people trust); and (2) that more essential parts of life (such as education, as opposed to television) are becoming rapidly more expensive.
(Not stating which of his points I agree with, but I think that "private school enrollment is declining" does not necessarily rebut the core of his arguments.)
* Though I think my meaning was clear
** Though of course this becomes more complicated when you look at figures for all private schools