In short? I don't think they'd be able to. They could pretend to. They could even make it seem like a character died, but then they are just hanging just over the edge, hanging onto a convinient root. Besides, this isn't like one of those shows where if someone dies, they are never replaced, and never come back. This is a show based on friendship, magic, and ponies. Besides, they can't really replace these characters. In fact, they couldn't even kill off a side character. For example, Derpy. They were considering removing her, and look how people reacted. People love the characters too much for them to die in all honesty.