The question is indeed legitimate, but my overall concern is that the Fords' behaviour suggests they aren't interested in looking at the question dispassionately, but are instead intent on pushing their anti-intellectual, anti-poor ideology no matter what the rational answer might be. (And I think that usage figures indicate the rational answer is that libraries are needed now more than ever.)
But we had bookstores even before the "digital age", so why were libraries needed then? Just as not everyone can afford to buy books, not everyone can afford internet access. Libraries serve a very important purpose in the digital age by providing access to books and the internet for those who don't have other means. And that is especially critical in a down economy for getting the unemployed back to work.
And that's what's especially troubling about the Fords' attitude -- they do not care about services that are targeted at helping out the economically disadvantaged, even if those services might help turn such folks into employed taxpayers. Their perspective is small and petty and irrational.