This is really, really tricky, because the “right” default value varies widely from situation to situation. I don’t think there will ever be a one-size-fits-all answer for this.
Let me throw out some cases…
Defaulting anything to 0% would make a bad out-of-box experience for new users. The #1 FAQ would be “I’m playing music but I can’t hear anything”. That’s not an option.
Any situation involving an OS-provided mixer should default to 100% since the master volume control ultimately controls how loud things are, and the Roon volume control is an extra digital attenuation stage.
Many DACs have an annoying fault: at anything but 100% volume, they are not bit-perfect. These DACs can’t do DSD playback at anything but 100% volume. We do want that to work out of the box, so that the user isn’t discovering their DAC’s limitations at the same moment when they’ve just installed Roon and then blaming us for it.
Any source component should probably be at 100%, since the volume control is likely to be downstream at the amp and we don’t want to apply extra attenuation.
The one situation where we totally agree that volume should not default to 100%? Any device with a built-in speaker.
Unfortunately, AirPlay doesn’t (to my knowledge) inform us if the device has a speaker or not, and setting the volume of the most common devices (AppleTV and AirPort Express) to <100% is not great, since they are source components that should have downstream volume controls elsewhere.
We can determine that with Squeezebox devices, and in fact, we default the volume of the Boom + Radio to 25%.
The transporter is a source component, and has a digital output–which is why it defaults to 100%.
It would be a better world if all devices could report their signal path to us. We would use the information to do good.