Funnily enough, I think that economically speaking, the answer to the first question changes over time, which is why there are sales at all. It's supply and demand - demand decreases over time as more people already own whatever is being sold and no longer need one, so fewer people buy it. Eventually, you reach a point where decreasing the price brings in enough new sales (either cheapskates who were waiting for sales or people who just couldn't afford to pay full price) that it compensates for the decreased income per sale and amounts to more money per time period. That's when the sale happens. Permanent price decreases work the same way. Particularly when it comes to software, there's no additional cost for selling more units, so the effort can be put to improving the product or making a new one to start selling when the previous one has saturated the market.
In short, it's possible to wait for a sale, but then you don't get to play the game while you're waiting, and all of your friends will be talking freely about it by then. This is why I so rarely play video games anymore.