The main problem with opt-ins for most users is that if a developer releases a new module, then unless the user hangs out on Rack social media, then they will have no idea. It won’t appear in their browser when the plugin updates.
Having made an initial selection on the library, if a user wants to get anything new, they need to regularly go back to the library, check though all the ‘partial’ plugins they have selected, see if there is anything new there and opt-in to that. Ain’t gonna happen.
From my discussions with users over the last couple of days, they had no idea that was the case. Some of them didn’t even realise that they could subscribe to entire plugins because of the way the library defaults to individual module subscriptions. Some users entire libraries are based on module subscriptions only. They will never see anything new in their module browser.