We're talking about old model vs new, and what is better for the writers. Yes, in the new model authors would have to work on alternate money streams. That doesn't mean that the new model is better. Certainly you can understand why writers would be concerned.
The reason that this is not zero sum is because _the amount you are willing to spend on books changes as a function of how much you like to read._
If an author like Scalzi who talks about other people's books gets you to read more, there's a good chance that you will keep reading more. Instead of being willing to spend twenty bucks a year on books, you're willing to spend a hundred. The spending limit is higher, so there's more money for all authors to compete for!
Maybe an author could get you to spend the rest of your 'spending cap' on merch (although that strikes me as a terrible idea, given what happens to webcomics who try that model) but the actual money in the publishing game has gone down. And encouraging a reader to read somebody else results in less money to you. Zero sum.
And yes, 'the amount of money in the industry' is exactly what's at issue. Scalzi is worried about being able to make his living writing. That's pretty reasonable.