In answer to the OP's question, at least where AC is concerned it depends on the route. On some routings (YYZ-GRU, for example, where AC has the only direct flight) the J cabin often sells out completely two weeks or more in advance of the travel date. That flight has always been very difficult to upgrade on, via LMU or ecredits/certs. YVR-SYD, YYZ-FRA, YYZ-MUC, YYZ-ZUR are further routes that are historically hard to upgrade because they are heavily populated by travelers paying for J to begin with.

Although we as FF's hoping for an upgrade might like to believe otherwise, the majority of AC's J seats on a majority of routes are, in fact, sold to paying customers - a mix of wealthy leisure travelers and business travelers with the ability to expense riding up front (along with AC staff, mainly commuting on domestic routes). There are obviously some exceptions to this, but system-wide, I do believe the vast majority of J passengers pay to put their butts in the big seats.

As time goes on I believe fewer of AC's J seats will be occupied by upgraded FFs, as miles become (1) harder to earn, and (2) increasingly expensive to redeem. Even someone who flies daily will simply have fewer opportunities to redeem upgrades. The glory days are over.