But "Why does it need to exist?" kills every art project, side project, or anything built for the love of building. None of those need to exist either.
There's also the argument that if something isn't forbidden, it should be allowed. The way you lose rights over time is by your existing rights getting clipped off at the edges.
A lot of the criticisms leveled at me here would also apply vs Google back when Larry and Sergey were grad students. If they'd had to worry that indexing content equals copyright infringement, they'd have a steep hill to climb.
> There's also the argument that if something isn't forbidden, it should be allowed
That's a really fun legal argument but a pretty irrelevant ethical one.
No, they are not "replacement frontends". Those are known as "3rd party clients". These are not clients but clones. They are cloning all the content. That means they've replicated the backend, the frontend, and the database full of content, by scraping/pirating it from the original source.
old.reddit.com is an "alternative frontend". Clones of scraped content are not.