Regarding this "don't upgrade", is there something that prevents users from downgrading to previous version, if there are problems? (like firmware upgrades are done on modern games consoles, for example)

Also, an average user will probably think it's better to add as much features as possible to the firmware. But is there actually any benefit in doing that, instead of "doing as little as possible", and handing the control to the OS as soon as possible? (I assume the answer is "absolutely none") - would the OS actually run exactly the same, if firmware was completely erased once the system was booted up?

I think it's also good to point out that this firmware rom is mostly different from Amiga Kickstart ROMs. Both definitely do some initializations, but classic Amiga ROM (or ROM image in RAM) continues to play a significant role once the OS is up and running.