I think that we are facing the best time ever to invest in PMs for two main reasons. First, world oil production, in the least, has plateaued (it may already be on the downside of production), and due to lowering EROEI, it will make mining more expensive. If costs rise, price will rise. Now throw in the fact that gold production peaked over a decade ago, platinum peaked last year (abouts), and that silver is being mined ferociously (and should peak within a year maybe), and see how it would be the best investment.
The best way to understand how declining supply raises price is to draw a supply/demand graph. Then shift supply lower. Now price is higher. Usually demand would drop, putting price in equilibrium, but.....that is for when considering standard behavior. Economics discounts peak oil theory (peak anything theory in fact, due to "technology" and other made up factors). I think that once the supply crunch happens, it will actually INCREASE demand. So now put that on the graph. Price has now doubled. This in a moment.
It is important to understand this window, but now remember this: prices of PMs have been suppressed by Major banks shorting them (here's looking at you, JPM). This was done to keep the dollar's purchasing power in a range where the world thought it was still a viable means of exchange. Once peak oil is realized, the window closes, and the window is closing.