The average salary for a Script Writer is $64,000/yr. If you're simply looking at the value they produce, that's bloody peanuts.

And you say they're replacable, but you could say the same thing about electricians, and they have a union and strike. Of course, the Electrician produces less value but makes on average $10,000 more than the average Script Writer ($75,000).

I'd also like to remind you to separate the end product from the quality of the Script Writer. They do a skilled job and work hard, and they are only one cog in a machine. I have had this exact conversation with managers and execs in my own field who wanted to hold individual contributors accountable for things that simply were not their fault.