That is true now, but 4-5 years ago I was starting a small ISP and prices were almost triple that. $80-$100/address seemed somewhat common IIRC, and prices only seemed to be going up. So I can certainly see an ISP looking at that and making a plan for a future where IPs cost $200+ each.
Since then prices have come down. I can only speculate that high prices caused a bunch of organisations to decide to sell their unused IP blocks. I suppose the question is: Does the market now have enough spare capacity to see us through to IPv6-only adoption at reasonable IP costs, or will we be seeing high prices again in 5+ years?
I doubt we will see IPv6-only ISPs. Such provider will have to face complaints that some part of Internet is not working correctly. It can be written it in the offer, but for not technical people some services will be broken by ISP.
It is core of problem with IPv6 adoption - from ISP perspective it do not solve any problem because they still needs solution for connecting to IPv4. Opposite way it works - if you have IPv4 there are bridges to IPv6.
There are IPv6 first ISPs. There are ways to do IPv4 from IPv6. MAT does IPv4 CGNAT over IPv6 with stateless gateways. 464XLAT is used by mobile ISPs but requires software on each device. It wouldn’t work for ISP, but there is NAT64 which works well for companies and networks that can control software.
It's not uncommon that 'security solutions' in commerce and the like just refuses traffic from IPv6. Sometimes I switch to a rather dirty IPv4 address to feel something when I bypass it that way.
The solution is already here. ISPs roll out CGNAT for IPv4, but their networks are dual stack, so the client devices have real IPv6 connectivity as well as IPv4 through a NAT.
It's not great, but for 99% of consumers, they can't tell the difference. The ISP only needs a small 28 or 29 allocation, and with that they serve tens to hundreds of thousands of clients.
Your ISP doesn't do statics, but not because of the cost.