Er, because "they" aren't "them". If they die before seeing "all these things occur", then they aren't part of 'what' they think they are. There's that nagging prerequisite of being alive from start to finish.
It's quite simple actually. Don't need anyone to inject upon or further translate anything Jesus said cause he was talking to fishermen, housewives, and regular everyday ancient people, not 21st century nuclear physicists.