That's indeed the original reasoning for building in on-the-fly renumbering to IPv6, but it was supposed to still be a rare event.

But consider: (1) that for ages the route table growth has been slow due to v4 shortage, and router memory has kept getting cheaper on the normal memory cost curve for this plateau phase in history, and (2) Consumer ISPs have been abusing this and making prefixes jump around even on CPE reboots, instead of having them stable for months and years like is the design, and (3) mechanisms to smooth out renumbering haven't been deployed because ISPs have gotten away with doing [2] without any mitigations.

The 'net infra could probably afford to start giving many more people portable blocks, and it would be only fair that ISPs would have to pay a bit for their sins regarding [2], and to this would add incentives to do [3].