A key aspect to understanding how 'forever' is possible on Arweave, is in understanding the storage endowment mechanism.
It operates similarly to a traditional endowment. The user pays a 'principle' upfront which generates interest and the ongoing resources required to pay those who provide hard drive space for storage in perpetuity.
The endowment is based on very conservative figures extrapolated from historical data on the price of storing data, which has been decreasing at a steady and predictable annual rate of decline of 30%.
This data enables the mathematical calculation for a finite cost for indefinite storage.
For more details regarding the network’s economics, expectations and mathematics, please see chapter 3.2.2 of the yellow paper.