Wicked for example is literally a freakin' Wizard of Oz fic and it's one of the most successful musicals out there and it earned its fame. Eragon was basically LotR with dragons. As long as credit is given when needed and it's not a blatant rip off, then it's fine.

The thing about writing/literature is that by this point most everything has already been tried and done (seldom there is something truly unique) because writing has been happening since very early man in some form or another. So many modern stories are inspired by really, really old folklore. At this point, it's all about how the writer takes topics/themes/an idea and makes it there own.

On top of that it's very commonplace (and it always has been) for writers to take inspiration from other works. Art breeds art. Writing fuels new writing.

If the thing that took inspiration from another show gets more popular that's fine; maybe people took to the writing style better. Maybe it was simply the more unique take on a topic like 'vampires vs werewolves'. Maybe it's just a matter of opinion and there are still people who like the original thing more.

There's a difference between being inspired by something and flat out copying it. There's nothing wrong with getting inspiration or having similar ideas. Copying someone is uncreative and unimpressive.