All your points are true - but I think using the API to essentially just create a “better front end” for Goodreads would be the the new idea here because of that momentum. You wouldn’t need to add your books or friends on this new app, you would just login with Goodreads and it would all be there. It could also two way sync so even if your friends want to keep using goodreads, any ratings you put on this new app would still show up on your goodreads profile.

The reason “better frontend” is in quotes is because it wouldn’t stop at just making the UI/UX better, but also do better search using Algolia, a better book recommendation engine, etc. but there’s no reason it couldn’t just use the goodreads api as essentially the backend for user profiles and ratings