When most people refer to Open Source Software, they're referring to serious projects like the Linux kernel, Apache, PostreSQL, Firefox, etc. They're not referring to random crap on Github.
The serious projects are actually better than their closed-source counterparts, both in quality, and in their ability to be extended and customized. Customization is not going to be cheap or easy, but it will at least be possible.
The fact that I wrote a toy renderer in Rust five years ago and haven't touched it since then doesn't change this. The fact that there are 100 or 1000 times as many toy projects as there are serious projects doesn't change this either.