The point of having tapered rates is well established as being progressive and fair. Governments do this with income tax. The basic premise is that $1 means more to a person with an income of $10,000 per year, compared to a person with an income of $100,000, or a $1,000,000. It therefore follows that developers earning less money are not as able to pay as those earning higher incomes. A flat rate favours bigger developers.

If Apple wants to promote diversity, then they should have a progressive system, and lower the barrier to smaller developers.

This actually may prove to increase Apple revenues ultimately by helping smaller developers stay in the game over the long term because their business would not have been otherwise viable.

Adding income tiers is not that more complex.