GT 1 and 2 were on PSX and GT 3 and 4 were on PS2 so it makes sense that if both previous systems got two GT games that the PS3 would get two GT games.
If I was Sony I'd drop support for the PS3 like a bad penny and move all development to the PS4. But since PS3 games can't run natively on PS4 porting them would be a bitch so maybe it is smart to let anything already in the oven finish baking. But they shouldn't make it a priority and maybe even move the project over to a secondary team so the primary team can start work on GT 7 for PS4 so the wait won't be as long as it was for GT 5.