If you need to build a product that relies heavily on real time updates, I would look into using Elixir and Phoenix.[0] They nailed the channel abstraction which is the main entry point for realtime communication over websockets. It takes me hours to make scalable realtime applications in what would normally take me days using other systems. The language may take some time to get used to, and the ecosystem isn't as mature as other languages, but what is there is incredibly impressive.
Firebase does a lot more, including a slew of Auth options that make life much easier.
Add to that the ability to resolve connections dropping out (common on mobile) and that their libraries have been ported all over the place, and Firebase is a defacto answer for mobile developers. It can be up and running from in less than 30 minutes for someone who has 0 experience in cloud development.
The common use cases for firebase can be easily reproduced with Phoenix. Phoenix also comes with a handy presence feature that allows you to track whether someone is currently using the product. (Think which present users in a chat room)
I understand the skepticism, but I would highly suggest taking a look and playing around. It's really, really good plus you get to fully own everything you build ;)
Also, "fully owning" everything isn't a selling point for everyone. Some don't want to own the uptime, manage infrastructure devops etc. Serverless/managed services have their use cases.
Small teams, individual developers, bootstrapping an app quickly, running a web app with no servers to manage... Often times much more valuable capabilities than being able to reimplement to functionality already availabile to you for very low cost.
[0]: https://phoenixframework.org/