Oatek, I got the same offer (domestic only too) with same expiry on 15 Dec. Qantas must have worked out (not hard) that almost the only domestic flying I do is to ADL in March for the Adelaide Fest and Womad, with a very occasional BNE jaunt. Otherwise it is to NZ 4-5 times a year and maybe Europe twice a year. So I do need the 50% SC offers on those (only in economy) to punt along continuation of Gold. I have however taken it into my own hands to upgrade my coming trip in a few days to AKL from econ to F on QF/EK (a first). Darn thing cost me $100 for the change alone of course, but F was on sale at only $899 one way (J was $749), which is normally about $1350! So spending and extra $550 on top of $430 to get 120SC rather than 25SC was "worthwhile" (lets face it I normally have to overspend to get SG).
The last targeted 50%SC offer I got was in May 2016 when I was already set to coast into Gold with 605SC with a final trip to NZ about then. So it was useless then - why overspend on another flight and useless SCs just to have the SCs vacuumed away at the end of my year June? They even "comp'd" me Gold as i was so close at 565SC (but the return econ to AKL in May took me to 605). The vagaries of that QF offer computer! The F trip to AKL will take me to just over 400 so getting another 200 to retain SG is not that hard in 6 months, regardless of their domestic offer.