Hi Dance
There is a point that you aren't take into consideration, you could sell your hash at any moment, so even with GHS prices falling if they take more than 254 days to reach 0 you will have that sell as an earning.
Depending of difficulty increase, BTC prices and so on you could made profit.
Agree with you that is too risky with S4 at current prices but isn't impossible
Actually I've had put some buy orders that trigger with half prices and already traded some S3 hash with very good profit
^This^
Not everyone has a buy and hold mentality, they play it on a day to day basis based on daily yield, and then trading back on the market.
I agree it may not be the best investment in the long run, but there are ways to speculate, and still make profit. Although with BTC prices staying around this $230 level, and the difficulty rising faster those windows are closing on the S3 and S4 in Hashnest.