For context, 2000 concurrent connections would be quite a large site. If you're hitting 2000 concurrents, $500 probably wouldn't be an issue for you. It's also worth noting that Firebase employs burstable billing at the 95th percentile, so only sustained overuse within the monthly billing period will result in a surcharge.
As for why we charge for connections in general, they do tend to be the most expensive thing to scale. They're also useful as a proxy of how "big" a site is (in terms of users). They're kind of the analog to "page views" in today's world of single-page-apps that update in real-time
As for why we charge for connections in general, they do tend to be the most expensive thing to scale. They're also useful as a proxy of how "big" a site is (in terms of users). They're kind of the analog to "page views" in today's world of single-page-apps that update in real-time