Thats a real big subject and if one is used to bitcoin then its like the 4th dimension of crypto. SAFEcoin is used to pay for storage. The coin is recycled and available for reissuance
While the total possible coins is 2^32 (approx 4 billion) the amount of coin that will be issued over the years will be many times that 4 billion because of recycling when people pay to upload data.
Farmers are rewarded according to farming rate (FR).
FR is calculated in real time according to the amount of available space in the network
As the available space drop FR rises which results in farmers being paid more often
So while plenty of available space farmers may get rewarded a coin at the rate of 1 coin per 10 thousand chunks retrieved (GETs).
And when the space drops they might get like 1 coin per 5000 GETs
Then as the avail space (compared to used space) drops further the rate increases.
The algorithm current will reward farmers at the rate of 1 coin per GET when all the available space is used.
After that the average number of copies will drop, but the network is still safe and data secure. And the farmers are rewarded one coin per GET. I doubt many capable users will resist those payment rates. That is why I doubt it would actually get to that point and I am confident that people will increase farming/vaults to take advantage of the great rates.
It is expected that farming rate will settle to a point where people are happy. If not enough then some will stop and the rate rises to a point where enough are farming to keep it about there.
As to world wide events killing off SAFE storage, then I doubt data store will be our biggest problem. Also I imagine that SAFE will not be the only store for critical data, which is only good backup/storage management.
Also there are a number of advancements coming out that allow massive backup storage in small devices (eg optical storage) and if the archive nodes used them we could likely have a archive store of every chunk in them. If that was done then even a world wide event would not kill the data but just require the network to come back up