#1: "Well if you buy a book with a credit card, the bookstore has no guarantee that that charge actually settles successfully."
#2: "Most people won't use Bitcoin directly. They'll keep their Bitcoin with a trusted provider. The trusted provider can verify capability to pay as quickly as a SQL transaction, via any mechanism mutually acceptable to it and the bookstore, which need not involve the blockchain. Providers which routinely allow their customers to steal from other members of the ecosystem won't be trusted for very long, and be forced to transact through the blockchain versus being being extended instantaneous credit."
The response to #2 is typically "We could call those providers 'banks'" and the Bitcoin community is, how shall I put this, divided by how much they like that outcome.