>But then they can't take bookings through google assistant, which is going to lose non-trvial amounts of business.
If it's costing more money than it's bringing in, then it's no longer worth it. If it's bringing in more money than it costs, then it's a good thing for the business.
If a business needs to hire a dedicated phone person because they are getting so many appointments filled, they aren't going to be upset. But if they get so many flakers that won't show up, they are losing money as customers that will show up are getting pushed out, so they will block Duplex. There are also other ways to solve this problem, require a phone number and name and block or charge people for missed appointments if they try to reschedule. Require some kind of down-payment over the phone when making the appointment. There are tons of solutions to this problem.
At no point is "Pay google to handle the calls" an option. This is really only for places that don't have an online appointment system (that possibly integrates into google), so the solution to the duplex calls would be to invest in one. Since "pay google to handle duplex" would look a lot like an API to a scheduling system anyway, and an independent one (with integrations into Google's systems) would reach more customers.