I've come across many screenshot posts on .win from several years ago where the post's content is no longer available, making it impossible for me to see what the post was about. So no, screenshots are not forever.

Since there are some advantages to both, then I would argue that people should provide both a screenshot and an archive link. It doesn't take much time at all, and that way you get the best of both worlds.

There are alternatives to wayback machine, and you can also download the archive to your own computer if you're worried about the website taking it down.