SAA did give up all their 74s quite some time ago, went to 340 and 330. The reason they sit at LHR all day is the problem of the North/South route and only a one or two hour time change. European carriers mostly have no change of time zone and used to go out night and return day but as the Biz traffic increased, Biz did not want to waste a day and tourists neither. Consequently everyone pays for the 12 hours (or so) of parking time. One of the reasons the BA's ex JNB+CPT arrive at LHR so early in the day is to release them for their next rotation.
Thread drift
At JNB during the day, they tow them into a long line for cleaning and servicing: BA (often x2), AF, LH, TK and others at various times. CPT also has BA parked up during the day. I have done daylight returns from both CPT and JNB at various times on BA, VS and old Sabena.
Other North/South routes all around the world have the same problem when down route.