It'll work forever, you just won't make back your investment that's all.
I don't understand that, it will keep making coins forever.. but I wont get my investment back?
Think of it just like a gold mine. First you want to make sure there is more gold in the mine than what you paid. If the price of gold is $1400 an ounce, and you paid $140,000 for the mine, it had better produce at least 100 ounces of gold. So you retain a geologist to estimate how many ounces of gold are in the mine. If it is less than 100 ounces then you should not buy the mine. Now you can keep digging, but after you mine out the rich veins (low difficulty) you will get less and less out of a days work. After a while, it will cost you more to dig than you get back from selling any gold you manage to find.
Some people might say "but gold will rise in price and you will make money that way!". If that were true, then you should just buy gold. That way you don't have to bother digging for it, you know exactly how much you will have, and you will pay a cheaper price for it than overpaying for a gold mine.
Many years back, the gold companies knew they could use leeching techniques to extract minute quantities of gold from huge piles of dirt. Back then it was abandoned as too expensive. Now that the value of gold has gone up, they are starting to use these techniques to get that gold. Through inflation, it now cost more to leech that gold than it would have when they refused to do it, yet the added value makes it worthwhile. The major mining concerns care about fiat, it's only certain individuals who care about holding the gold, mainly due to the amount needed to invest.
The gold will get mined sooner or later, it just depends on the cost vs the reward in Fiat. The same is true of BTC. Few people are able to buy and hold when compared to all of the people mining. The paying BTC to mine more BTC only works for certain individuals. To repeat what has been said to me... You can lead a horse to water.......