It's always going to be in Amazon's best interests to reward top sellers. All Star bonuses are super cheap compared to the percent of the pie the people who get them are getting. I just redid the math, and the top 100 authors (as in the 100 pen names that got an Author Bonus for any given month) are getting about 6-7% of the total pages. All Star bonuses are only about two percent of the total KU spend.
It's a lot cheaper to motivate people with All Star bonuses (it also motivates people who aren't earning the bonuses, and the gamification likely encourages people to stay in the system) than it is to increase the actual payout.
Please do correct my math if it's wrong. There are a lot of zeroes in there. I did make some assumptions about average earnings (20 million pages for top ten, 12 million for next ten, 7 million for next thirty, 4 million for next fifty. That's reasonably accurate IME. Also, I already closed the window where I did this, so these numbers are from memory).