Sadly, any Goodreads competitor will need to miraculously gain the network effect; everyone you know is on Goodreads, so it'll just be you and whoever you can convince to move to a new platform.
As for the downsides for Goodreads, this blatant lack of moderation is troublesome. I also dislike that Kindle / Amazon are the only visible links to purchase books by default. Amazon already dominates the ebook/audiobook market, so I also simply dislike Goodreads due to their acquisition by Amazon.
They thing is, on another level: they're done. They offer excellent cataloguing and the ability to share your reviews and so on and so forth. What more do they need to add? A visual refresh every few years? At this point I'd rather they spend the money their customers pay them on keeping things running, not adding crap no-one cares about, surveillance capitalism anti-features, or whatever.