They could have cut the amount of girls in half and the content wouldn't have actually changed much because there was a lot of recycling in the sprite animations too. I cannot see much fun playing this game with a reward system this repetitive.
No, they couldn't. Sure, I would prefer it if there was more content in total, but cutting the amount of girls in half just for an extra scene would be a terrible idea for this game. In its original form, before DLC, there were 15 girls. Let's be generous and half that to 8. One needs to be the starter, one needs to be the final challenge. That leaves a total of 6 girls to be captured and used for the ENTIRETY of the core gameplay.
An extra CG is not worth that, it would cut way too deep into the gameplay because the random NPC encounters are by far the most boring aspect. Sure, you could use the same mechanics they put into the girls and instead use them for random dudes as boss battles, but that would diminish the feeling of finally conquering a strong girl that gave you trouble and making her strength your own.
It could be a matter of taste, but to me, the most important thing about H-games is how the H resonates with the gameplay. I don't want a generic game against random soldiers with CGs as mere "rewards". The parts where I fight the girls on the battlefield are at least as important to me as the scenes.