I disagree. By default, the copyright holder reserves all rights. For the website to display that content, they need a license. When the owner deletes the content, it disappears from the website, so no license is required. This is a "happy path" that allows an agreement like "when you delete your account, you revoke our license to distribute your content, and we will comply with our legal obligations".
But the real world of production computer systems is not always the happy path. Consider the case where you delete your account, then the service suffers a database problem, and your content doesn't actually get deleted (failover to a replica, restore from backup, etc.) Now they are violating that contract with you. That is why they make the terms of the contract "in perpetuity" so the engineers can fix the production system incrementally, rather than saying "welp, there's a risk of a lawsuit if we failover to the backup, so we're shutting off the website and paying back the shareholders, everyone go home, we're done here".
Like I said, some companies take a middle ground where they really do guarantee deletion even if unhappy events take place at exactly the wrong time. It is possible. It's just not very economical.
But the real world of production computer systems is not always the happy path. Consider the case where you delete your account, then the service suffers a database problem, and your content doesn't actually get deleted (failover to a replica, restore from backup, etc.) Now they are violating that contract with you. That is why they make the terms of the contract "in perpetuity" so the engineers can fix the production system incrementally, rather than saying "welp, there's a risk of a lawsuit if we failover to the backup, so we're shutting off the website and paying back the shareholders, everyone go home, we're done here".
Like I said, some companies take a middle ground where they really do guarantee deletion even if unhappy events take place at exactly the wrong time. It is possible. It's just not very economical.