Perhaps all this dynamic IP range nonsense is intentional, not just due to insane BRAS / BNG devices. It could be that these big domestic ISPs want to make sure that business users won't go near these deals and will have to choose something more expensive to get the addressing they need. For example, to be able to write firewall rules with known IP ranges mentioned in them, for site-to-site stuff too. My few mutually trusted ‘sites’ allow access from each other's IP ranges through static firewall holes, for example.
I do think it's a shame for future application developers though as everything like this makes it a bit more difficult to deploy new sexy kinds of IPv6 applications. IPv6 needs to be a clean break from all the bad practices and design mistakes made with IPv4. No NAT, no assumptions about single IPs only being assigned to an interface, scopes defined, link-local addresses explained properly, PMTUD, reliable zeroconfig, all those many good things.