If AC ever came out with a similarly priced SYD Latitude Pass, I'd be all over it. But problems would arise elsewhere. For example, if YVR-LHR does not have any upgrade space available, you have lots of alternatives, such as connecting in YEG. That translates into lots of different flight options every day.
But on YVR-SYD, there's only a single option, and either there is R space available, or there is not. Not alternative options. As well, R space seems to be getting tighter on this route.