So an old guy is not old, he is slacking at youth... ok. Anyway, you differentiate them by rating points, as I've previously stated. No need to differentiate them any further.
As I've said teaching and writing is meaningless for their title. They are not necessarily teachers, some of them just want to play the game. Some of them will never write a book as well.
As for rates of those who actually teach, you can always check their FIDE profile and decide if you wish to pay their rates (if that is what worries you). I mean they can easily say they want 1 000$ per hour regardless of their title, it is your decision if you want to pay that or not. And one more thing - higher rating doesn't necessary mean a better teacher. There are people rated 2 000 who are better at teaching than 2 700 GM, it is a completely unrelated skill.
"People just wanted to differentiate those who are at least borderline candidates for world champion so they invented this expression"
That's my point. We should invent a way to differentiate the slacker GMs from the real GMs. Otherwise, you will get people who just play to become a GM and then stop playing. They will write books and charge $100 for lessons just because they achieved something once.