> only the people who opted to be forgotten benefit
That is not true. One of the main points of GDPR is that you only store data as long as it is reasonable for your business. If I haven't been a customer at a store for 5 years, I can fully expect that my data has been deleted. How long reasonable is will be determined in the court, but I expect it is going to be in the customers favor.
Exceptions for slander and copyrighted information (e.g. photos) that were posted by other users? (or even regular content posted by the user that they 100% own) Absolutely -- but even in these cases, requiring that the third-party platform owners track down all potentially offensive content (much of which may be posted by other users) and then purge every layer of their backups after removing the offending content is overkill and shifts the responsibility away from the offender (whoever posted the content, even if it was the user himself) to the platform owner.
Ultimately, the ability to un-publish content you own that you published should be sufficient regulation-wise, and most popular platforms already enable that as a basic feature.
It's not about undoing participation in information sharing. GPDR does not require Wikipedia to delete everything I've contributed, but the fact that I have contributed is subject to be deleted if I request so. Furthermore, there must be explicit opt-in, and that is a good thing.
It's a trivial problem to purge from the backup, really. It's not a hard problem to solve. Many companies have done so before, and now we might even see standard tools to do so.
There is really no perfect way to do it, and it speaks to how unreasonable 'the right to be forgotten' actually is. When did people ever have a right to be forgotten? No one can force me to forget embarrassing things I've witnessed others do, and vice versa. The same goes for my local copies of photos, videos, diary entries, etc. of such events. Can't make me destroy those love letters you sent. You can't instigate a county-wide manhunt to destroy copies of every local newspaper that covered you in some way you want to forget. You cannot delete most of your public records in real estate, justice, business, etc. You cannot start fresh on your credit history whenever you want.
Sure, there is more easily accessible data on the average person with the Internet now, but the majority of the types of data already existed in the pre-Internet-era, perhaps with the exception of ad profiles. Consider all of the political scandals (particularly ones that happened long before they were unearthed) in the pre-Internet era.
So, what about when companies have data breaches and expose your data? I think they should absolutely be held liable. But it should be their choice to choose to be liable and determine how much they would need to forget to avoid liability. Furthermore, all of the active users are still compromised with GPDR -- only the people who opted to be forgotten benefit. So instead, there needs to be a framework for addressing data breaches and poor security, rather than deleting every trace of a specific type of data profile that a user and/or associated users posted on your platform.