They haven't been fired, so I don't know why you think the studios see them as replacable.
In fact, many shows have grinded to a halt without them. Jon Stewart offered to pay his writers out of his own pocket just to stay on the air.

Potential wages mean nothing. Just because you earn a percentage of sales and doesn't mean you will earn millions. Sure, if many many millions of people bought your dvd then fine, but that's not really likely. There are many things involved beyond the script including the actors/marketing budgets/editing etc.

To be honest, it sounds to me like you know nothing about this and you're basing your opinion on pretty naive assumptions