Ever thought those rank 60-99 areas and mobs are there because the level cap will some day be raised? Would you rather SE have put up invisible walls and closed off the areas completely or let you explore them? They aren't meant to be SPed on right now.
If you want different mobs that's fine, SE should be adding more mobs of varying ranks. Lower ranked beastmen, lower ranked goobbues, lower ranked snurbles, lower ranked ogres, lower ranked plasma balls, all those cool mobs that do exist at higher ranks. As well as completely new mobs.
Additionally the game should be balanced where a mob of rank 65+ completely destroys a full party encouraging people to fight challenging mobs of around +10 ranks. They may already be doing this, who knows?
There's no logical reason why someone should want to fight rank 99 mobs for SP. Rank is just a number, there's other ways of increasing the difficulty and variety of fights that A) Make more sense and B) Can be balanced easier. Fighting a rank 99 goblin is no more fun than fighting an adjusted rank 65 goblin.
"Please make our mobs challenging!" is what the complaint should be, not "Please, sir, I want some more SP!".