You might not be able to retire from BC mining. However, if computers are your hobby and you're buying equipment that you'd use for other purposes (e.g. gaming) where is the downside? Just don't buy equipment that you can't afford, and it'll work out.
It only works out if your BTC income is greater than your lecky bills, if it's not then you are better off leaving your PC off when its not gaming, unless mummy and daddy are paying your bills, in which case you could indeed leave it on 24/7 and profit no matter what. We will soon reach a point where current gen Nvidia GPU mining is no longer profitable for instance. My GTX580 is a loss maker once difficulty goes over 2,000,000 which could well be within the next 4 weeks. 2m probably renders most, if not all Nvidia mining loss making.