Supply is not decreasing fromthe miners, in fact I would argue that practically all mined coins (25 each 10 minutes) are being sold these days instead of being horded. If you run an expensive mining rig, you want to sell as quickly as possible. In older days, there could be some miners who would mine for their own use. Now, casual miners are a tiny fraction of mining production.
So, there has to be roughly 20K USD of new money coming into BTC economy every 10 minutes to keep the current price. Is this sustainable? I do not know, but you definitely need new blood to keep it up.