your outgoing traffic will still be fine
That may not be fine as well, since unless IPv6 can cram both host IPs into the packet, existing sessions will get dropped (which may not happen with IPv4, since IPs stay the same). Also, that requires more complex firewall configuration (what's the probability that one of the IPs will not be entered?).
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.
There are a few problems with this:
1. The ISPs must be willing to cooperate (unlike now, they only have to provide BGP access).
2. The customer still cannot change ISPs (now I can take my AS to another ISP if I do not like the current one or another pair of ISPs if I'm moving and the current ISPs do not provide service in the new location).
3. The failure of an ISP must trigger a BGP announce to stop traffic from coming to it. This may not happen. Currently we had multiple problems where the main ISP failed but did not announce that - out BGP router still though that the ISP is good. I had to write a script that checks if the internet is accessible and if not (for a few minutes) forces our BGP router to use the other ISP (done with prepends and priorities).