Google should frankly just quit indexing Pinterest until Pinterest rejoins the open web. That goes for any registration-mandatory or paywalled site. Looking at you, Medium.
Now Medium is a different story as the content is mostly original.
I see your point and I think you understand the product better than most in this thread but I think you’re minimizing the value of enrichment that “pins” over images or static links provide. In the same way, the comment section enriches news articles on hackernews and this site is worthy of indexing by Google.