This is true only for residential ISP's. Business networks often have many users on a single /64 and those users are often unrelated to each other. The same goes for mobile networks. To preserve the collateral damage-limiting benefits of IPv6, it is necessary to keep addresses separate. I would, however, support sharing Echo or talk page notifications for all addresses in the same /64. It is already possible to create a talk page in the form of [address]/64 so this could just be implemented as sending notifications on editing of that talk page.
A better proposal than consolidating the contributions pages would be to add built-in support for range contributions, a feature long desired even for IPv4. Even for home networks, putting everyone under one /64 is a poor design decision which basically negates the collateral damage benefits of IPv6.