Keeping a secondhand bookshop going is a marginal enterprise anyway. I doubt the charity bookshops affect them much. In Edinburgh one of the largest private second-hand bookshops is a couple of minutes walk from two specialist charity bookshops and about five charity shops with large book sections.

Drastic cost cutting by supermarkets and "airport" style book retailers is more damaging. But it only affects some kinds of book, things like paperback fiction and military potboilers. Many nonfiction genres are still reliable sellers, if the seller actually understands what they've got. (I've taken the trouble to learn what's what with books on religion, and I can sell a lot of them. Hardly any second-hand dealers bother).