Yeah, that turned out to be one of the big problems with IPv6 address aggregation - sounds great in the ivory tower, doesn't meet the needs of real customers, which is too bad, because every company that wants their own AS and routable address block is demanding a resource from every backbone router in the world.
IPv6's solution to the problem was to allow interfaces to have multiple IPv6 addresses, so you'd have advertise address 2001:AAAA:xyzw:: on Carrier A and 2001:BBBB:abcd:: on Carrier B, both of which can reach your premises routers and firewalls, and if a backhoe or router failure takes out your access to Carrier A, people can still reach your Carrier B address. Except, well, your DNS server needs to update pretty much instantly, and browsers often cache DNS results for a day or more, so half your users won't be able to reach your website, and address aggregation means that you didn't get your own BGP AS to announce route changes with, but hey, your outgoing traffic will still be fine.
My back-of-a-napkin solution to this a few years ago was that there's an obvious business model for a few ISP to conspire to jointly provide dual-homing. For instance, if you've got up to 256 carriers, 00 through FF, each pair aa and bb can use BGP to advertise a block 2222:aabb:/32 to the world, and have customer 2222:aabb:xyzw::/48, so the global BGP tables get 32K routes for the pairs of ISPs, and each pair of ISPs shares another up-to-64K routes with each other using either iBGP or other local routing protocols to deal with their customers actual dual homing. (Obviously you can vary the number of ISPs, size of the dual-homed blocks, amount of prefix for this application (since