Long haul in economy has always sucked. I doubt it's a SAA thing, having flown a number of airlines between Europe and Southern Africa. And yes, there is generally more leg space in the 738 as opposed to the 747 (economy). That's why after 12 hrs all the economy passengers look longingly at the seating arrangements in business as they exit. Even the budget airlines 737's have more space. Try flying longhaul with one of the holiday airlines, then there's even less space.
I wouldn't moan about it this way, and you can always vote with your wallet.