I'm of the same agreement, although somewhat partial to the IXP option, as it would force ILEC types to get better about routing to them. My point is that it's reductive to boil it down to a single number when it's going to vary significantly based on the endpoints the user chooses to communicate with - the 100ms target is great for a destination on the other side of the globe, but horrid for a destination in the same city.
I feel like it's obvious that it would be measuring local-ish latency. That's not reductive, it's separation of issues. If someone wants to communicate across the world, their total latency should be a consistent number added to the local-ish number.
