Reddit gains an irrevocable, sub-licensable, royalty-free license for user-created content posted on its site.
You mean Reddit claims to have an irrevocable royalty-free license. Based on an assumption some clause they stuck in the fine print of a user agreement would be upholdable against every user of their website - including for completely unforseen users by users who posted them 5, 10 years ago, etc --- Including some users who are unaware of the terms at the time of uploading items to Reddit's service, And some users who are under the age of 18, and therefore lack the legal capacity to bind themself irrevokably to a contract at the time of submitting some content.
And Likely based on an ignorance of 1. Termination of agreements including irrevocability due to some contributors having passed away. 2. the Termination of Transfers and Licenses Granted Under 17 U.S.C. 203 - Which allows termination of a grant of copyright license after 30 years under set conditions regardless of a prior licensing agreement.
As soon as 10 years from now: There will be Reddit users who will have been posting to Reddit for 30 years, AND there will be some users who could issue Notices of Copyright License Termination for all rights from Reddit under the grants termination clause. As soon as 20 years ago there can be users asserting a revokation or that their license grant is invalid was not valid at the time content was submitted, because they lacked legal capacity - being a Minor, or being unaware of the attempted copyright agreement, at the time of writing that content.