Serving a subdivision of 1,000 homes with 95% phone service costs about the same as one with 5% service -- the trunk lines and switchboxes still need to be there. But the revenues are 19x smaller.
And it's likely that the remaining customers are _not_ higher than average on the socioeconomic status. Though it seems people _will_ drop landline service rather than mobile, when push comes to shove. Landline subscriptions plummeted during the 2007-8 financial crisis, mobile rose.
Generalistic and moderated instance.