The cash amount is the cash amount. It doesn't go backwards; it is the only thing that is constant and directly based on ticket sales.
What you are seeing is the annuity amount getting better, because of better interest rates and Mega Millions going to a 29-year (30 payment) annuity with a 5% annual increase.
So the cash amount just keeps increasing based on actual ticket sales, and the annuity is calculated based on whatever cash is in the pot.
Lots of people base their ticket purchase decisions on the amount in the cash pot, there is nothing wrong with that. But you need to understand that the cash value is not calculcated based on the annuity. It is the exact opposite: the annuity amount is calculated based on the cash on-hand (and estimated to be on-hand just before draw time).
If the cash value is not high enough for you, then maybe you should complain that not enough people are buying tickets.