Does having fewer ingredients make for better recipes?
Love that Thomas Vail!
My advice is similar to that in that people often think that the fewer character the tighter or better the story but it's more a quality over quantity type of thing. Your characters/ingredients should be of a high quality but that doesn't equal having fewer means they're better. If you're having trouble killing them off I'd say you're in the right direction for quality there.
I'd argue fewer characters are easier for the reader to follow which is why some writers make a conscious choice of limiting how many they have per story. Thing is we're filling a world each time, your backdrop most likely has a billion characters and they've all got interesting lives too. In a way I think I'm starting to answer your bonus question as well, in that you need the focus on your cast that completes your story because it is so easy to develop characters that have lives apart from the main story that are worth following. I find myself over developing characters out of the story I created them for and finding they're fit for their own story all the time.