This was a performance optimization when they did it for Gradle projects, there's a
long thread on the topic that I see you already participated in. I think Android Studio overrides this behaviour to this day, just ignoring the toggle in Advanced Settings.
Anyway, I agree with you, it's a severely degraded experience in a lot of ways, and I also always want sources to be available.
For Amper there was
this issue which made it possible to download sources, but I don't know what happened to making Amper download sources by default. I thought it was something we were going to do, someone from the Amper team can hopefully chime in on that part 😄