Definitely negative into WLG's short runway as a 777-300 would have to take off empty to get back to SYD. Not to mention fog,winds etc. which make it an operationally hassled place to fly.
Trans-Tasman is already well to over-served and yields are low. Add on sectors for EK etc rely largely on local traffic rights,code-shares and cargo, and it's unlikely BA could get a 'higher than SYD' yield for LON/NZ terminating traffic when the market is already covered with just 1 stop ex Europe. Operating on to SYD when there's little premium over LHR/SIN yields is bad enough without further complicating things. You're right though - it is a waste of expensive machinery and doubtless one of the factors in BA and QF's retreat from the market.
In general terms, tag on services seldom cover additional costs and if the onward destination can't sustain a dedicated aircraft, it's better just to interline or concentrate on point to point. If the layover destination isn't full, then it's probably better to pull out.