Well, there is hardly anything left to write about. Let's just say you have an idea about your niche and you decided to search for any reference about that idea online, but to your surprising someone have already written about it somewhere else. Now would you like to drop that idea or just go on and write about it? Of course you should write about it.
In this World nothing is new, everything is just an alteration of something else . Just use your words and mix your style while writing about anything and it would do the job effectively.
Yes nothing is new and this is frustrating. Also remember that having sources cited adds to your credibility as a writer and this is really good.
Yes of course. It would increase writer's credibility. But sometimes people forget these small things and we should treat them as such. They should not be tagged as 'plagiarism' devil. They are just learning by doing these small mistakes.
I have even forgotten at times so yes I agree with you.