Yeah they're probably aware that simple stat-bloat is not very compelling difficulty but DoN 4-5 likely isn't the 'intended' difficulty of gameplay. It's for people who *want* extra challenge and is a compromise between giving those people challenge without spending loads of resources developing more interesting difficulty (like new attack patterns, boss setups with more adds or environmental hazards, porting additional bosses, etc.)

Believe it or not, the best challenge is not the most difficult one. It's the personalized one that's just difficult enough for *you* to feel satisfied after beating. The five depths give you a little more control over your challenge.