That's all true, but as you know, a swamped canoe on moving water is both a hazard for the paddler and a potential disaster for the boat. I think added flotation is worth whatever room is available, if those are likely concerns.
True..Moving water is different. But I suspect most of us in the East are dealing with lakes. I paddle far more on lakes than on rivers and pretty much portage rapids especially solo. In the West you have a different paddling ecosystem.. I think the original question was how to keep the boat from sinking.
Pinning potential on moving water requires a complete rethink.. how to you pack for a wilderness trip( dry bags do hold air with gear) supplememted with air bags..all caged in (or not-rescue being difficult with lashed in gear..but do you lose your gear if you swim?