You don't waive your copyright. You grant a license to reddit to use your content.
Read the link, it's all there:
You retain any ownership rights you have in Your Content, but you grant Reddit the following license to use that Content:
...
This doesn't apply unless you got compensated for your work. Reddit can add anything they like in their agreements, but that won't hold in court.
The artist can withdraw consent at any time.
You are confused. What you are describing applies to transferring copyright, not for granting a license while retaining the copyright.
If things worked the way you described, free software, for example licensed through the GPL, couldn't exist because then the authors could always take away the users' rights by retroactively revoking their license. Fortunately, it doesn't work that way.
There are cases where artists withdrew consent and their work had to be taken down, and to my knowledge all contracts can be amended or cancelled, especially if they're exploitative like reddits. You have a right to compensation if they profit off of your creative work, be it artwork, music, or writing.
Reddit Migration@kbin.social – 233 points – 
You don't waive your copyright. You grant a license to reddit to use your content.
Read the link, it's all there:
This doesn't apply unless you got compensated for your work. Reddit can add anything they like in their agreements, but that won't hold in court.
The artist can withdraw consent at any time.
You are confused. What you are describing applies to transferring copyright, not for granting a license while retaining the copyright.
If things worked the way you described, free software, for example licensed through the GPL, couldn't exist because then the authors could always take away the users' rights by retroactively revoking their license. Fortunately, it doesn't work that way.
There are cases where artists withdrew consent and their work had to be taken down, and to my knowledge all contracts can be amended or cancelled, especially if they're exploitative like reddits. You have a right to compensation if they profit off of your creative work, be it artwork, music, or writing.