The needs for dual-homing and failover were completely glossed over in the IPv6 design, and the IETF has not acknowledged these needs even to this day.
It's the same with IPv4, by the way. An ISP can assign you a block from their allocation, you can still announce it with your own ASN as long as it doesn't violate the issuing provider's policy, you're still a customer, etc.
And it's more of the "it's the thought that counts" thing. Multihoming was considered to be something rather exotic and mostly a requirement for ISPs or large companies.
The real problem is BGP isn't available from most home or "small business" internet connections. You can't multihome even if you want to. You need a DIA / enterprise plan, or you need to use tunneling (which is what I do, from several BGP capable VPSes, in a configuration that is too complex for the typical SMB.)
They thought about these things and put features into the protocol to support them. This is, once again, the opposite of your belief that they didn't.