I must beg to differ here. Only G, N, V, Q and T classes do not earn miles, and those are often exceptionally cheap or companion fares. Of course, you will see the non-earning fares in some markets more often than others, but to take the UK as an example even the GBP440 fare from LHR-SYD (20k miles return) is booked in W which earns full miles in most programs.

The remaining (even including W, as long as your program does not specifically reduce earning on SQ W fares as they do on BD, SK etc) all earn 100% of the miles flown.