There's a book that was written back in the early 90s called The Ultimate Resource, by Julian Simon. A fair amount of it is typical libertarian wishful thinking, but he very strongly makes the point that in a free society, there is no such thing as a peak. There are always alternatives, and if a resource becomes more expensive, we will start switching to them. If need be, we will also change our behavior to accommodate these alternatives.
To take a great example, look at gasoline prices. There is no shortage of gasoline in the United States. The price goes up, and suddenly fuel efficiency becomes much more important. As a result, economy cars are much more common, as are hybrids and electric cars. Consumption then goes down.
This would be the case even if gasoline were ten dollars or even twenty dollars a gallon. People would simply take the alternative options, which become more and more attractive as the "default" becomes more expensive.
Like I said, we are reached peak phosphorous as we mine it currently, and reaching peak energy as we currently use mostly fossil fuels.
And like I said, I believe we can use new technologies (like, solar power... or underwater phosphorous mining) to fix it.