Mining difficulty changes every 2 weeks, so that's not a problem. It's just that difficulty has to follow price, otherwise a bubble will be created (and it will explode).
You could see accumulating bitcoins more profitable than mining, but mining gets you actual brand new bitcoins and you don't have to sell them all just when you get them.
It is true but nothing to do with. Difficulty is normalized BY DESIGN for number of miners with current hardware to be constant and small FOREVER.
So that various new goods, services, technologies, etc, that people sell/buy for $, in real world, -> become just NEW ATI GPU cards in bitcoin world. Because in deflation scenario with constant number of society members, $/BTC exchange playing is always more profitable than ANY business, including even mining.