The supply of coins grows at the same rate regardless of demand. Stability requires a flexible supply increase to correspond with demand increase. This limitation will be an impediment to the growth of Bitcoin commerce.
You're thinking of the total in existence.
But what's important is the supply
for sale. That's what "supply" means in an economic sense. Just because an average of 7200 BTC are mined daily doesn't mean all those bitcoins will always be thrown onto the market. Miners have the option of holding onto and/or spending their bitcoins rather than putting them up for sale on the exchanges. Also, holders of bitcoins have the option of throwing their previously-purchased coins back onto the market for sale.
At any given time, between zero and all available bitcoins in existence can be for sale. That's as flexible as any other asset/resource/whatever.
You are correct if you assume miners hold when prices are low and sell when prices are high. But if most miners behave the same way as most of the market, they will constrict supply during uptrends and beef up supply during downtrends. In that case, supply moves in the opposite direction of demand.
Think about the original suppliers for gold and for Bitcoins. Gold miners mine more gold when prices go up. Alternatively, we can say that the earth supplies more gold to the miners due to the miners investing more mining resources into extracting it. For Bitcoin, the network gives miners the same amount of Bitcoins when prices rise. This remains true no matter the quantity of mining resources invested.
You made an important point here: "Gold miners mine more gold when prices go up."
But they do so by expending more effort and resources to mine more.
The equivalent, in the world of bitcoin mining, would be to apply more processing power to mining when prices go up, giving themselves a bigger cut of the block rewards.
And still, the point remains: the supply of bitcoins for sale at any point is as flexible as any other commodity (more than some others, really.) Even with all miners selling all coins ASAP, there are still 8 million bitcoins in existence; that's potentially about 22K bitcoins per day if sold off throughout the course of the year, far more than the miners produce.
So there's really no need to worry--some of these reserves will join the public market supply if the price rises enough, just like with gold, wheat, etc.