I can chime in with some very specific route experience (I travel YYC-YYZ heavily). In my experience, paid J consists of lawyers, bankers or individuals in the banking industry, and people involved in the oil patch in one way or another. I used to see some representation from union leaders (don't see that much any more). Sometimes folks in the media (CBC). Sometimes politicians. I would guess that somewhere around 2/3 of the J cabin is paid J. Obviously that depends on day of the week and time of the day, but that would be my guess for times when business travel is more likely (i.e. not at 4:00 on a Saturday afternoon). I think this is also specific to the route--YYC-YVR for example doesn't see a lot of paid J in my observation any more.

On international J, I leave that to others who have more experience, but would just say I would/will/have purchased J for leisure travel when the price is right.

Final observation: lets assume that only half the cabin is paid J domestically. I bet that AC has figured out that it is still worth it to retain 12-20 J seats in order to capture people like me who fly a lot and get upgraded a lot vs. having people like me do a strict pricing analysis on AC vs. WS, and potentially losing a bunch of traffic to WS. Put another way: would you pay a 5-10% premium on an economy fare (vs. the competition) if you were consistently upgraded? And is it worth it to the airline if you spend $30k+ per year?