By default that’s how it works in Discourse. When you post something on a wiki, the assumption and expectation is that anyone can edit it. That’s what a wiki is. And on any Discourse forum where you are the administrator, you can implement any rules you like.
Right! You own the copyright to your work. You do not own the right to have it published on a web site forever. If someone deletes your work from a web site–because for the past 20 years or more, that’s how everyone expects wikis work–you still own the copyright to that work. If having a copy or your copyrighted work is important to you, then you should print it out and put it in a safe place.
Also, Discourse wikis, like most wikis, do retain (almost) all versions of the document, so in most circumstances, you can see an old version to get a copy of your work.