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.
These days though, the FB API has been locked down and basically can't be used for growing your userbase anymore. Any new startups in this space won't have the social graph advantage that Goodreads did. Sad.
Once upon a time I bought something online for about $10 from what was a legitimate business with an address in San Francisco. About 14 months later they claimed I subscribed to some service and started making huge charges to my card ($150/week). Getting them to stop and getting my money back was an enormously stressful and difficult process.
That's why I have a knee-jerk reaction - not a cold, logical reaction - to online purchases from companies I haven't used before, and I'm not the only person like that.
I've had incorrect subscription charges on one of my cards, and while it was minorly annoying (a search on the card website, fill out a form, repeat one more time the second month it happened) I can't imagine a scenario where it would be "enormously stressful and difficult".
Your card issuer is required to respond when you report fraudulent charges, and if they don't you need a new bank.
I've had no negative fallout from purchasing a subscription, and they have very good privacy controls for those who don't want the social aspect (so you can basically turn off other user interactions in many ways). It's there when I need it, doesn't seem to spam me and I probably paid with Paypal (I use Paypal as a way to not give 3rd parties my CC info, a proxy if you will) like most things.
It may look old school and appear at a glance like it's not maintained, but it's updated and run actively, there are a bajillion people using LibraryThing. Logging in to look at my account, there's a link on the right for the latest news posted today about "The January ER Batch is up! We've got 2,960 copies of 89 titles this month." (early reviewer books) https://www.librarything.com/er/list
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.