I read and saved an article from 2017 on this topic when I took...

I read and saved an article from 2017 on this topic when I took out a trial membership with a stock review website a few months ago. It seemed like a good publication (not a recommendation) but I didn't ultimately go ahead with the membership.

The section "Second Airport no Threat" covers some of the concerns raised. The point made by the author is that airlines have only a few slots available. Therefore, passengers paying to fly many thousands of kilometres from overseas destinations are unlikely to be concerned with demanding a slightly cheaper but more inconvenient airport further away. The overseas arrivals are where the money is. Let the second airport have freight and budget passengers who aren't spending money in duty free shops anyway. SYD is the main attraction. I doubt Tullamarine would be too bothered by Avalon, or Heathrow by Gatwick, or Bangkok's main airport BKK by DMK. My credit card company has made nothing from me over the years. I always pay on time and it costs them more to service my account than it gets in fees. I suspect the type of traffic going to the second airport will be similarly unprofitable, or, at most, only mildly-profitable customers.

I'm not interested myself in SYD at this price but not for the reason of the proposed second airport.

I'm attaching the article here in the hope it helps clarify the issue.

Who pays for Sydney's second airport_ - Intelligent Investor.PDF