A lot of what you're suggesting is flawed. First, doubling the number of transistors does not equal to doubling performance. This is a fundamental problem with your statements. Secondly, Sony never suggested they wouldn't replace the PS3 after 5 years. In fact, everyone expects they will. They simply claim that it will be a 10 year product in the sense that the PSX was a 10 year product (and its support ending last year).
Secondly, in absolute terms the PS3 isn't very powerful. It is much less powerful than almost any gaming computer sold this year. It has about 1/8th as much system memory as a gaming computer would, a fraction of the fill rate, etc.
Finally, I would question whether the PS3 could possibly be a 10 year product, even if that was advantageous to gamers in North America. Primarily, this is advantageous to Sony investors, because they want to understand how the PS3 can make a profit. Sony claims they'll be making a profit in years 2-10, basically, and that's why the back end will be rewarding enough to stick with it. They're also trying to convince 3rd parties that games will be on sale for 10 years for this platform.
The PSX and PS2 were able to be long term products because of pricing and cost. They got so cheap, that they would be 1/2 or 1/3rd of the price of a new console when the new console launched and their costs were so low that Sony could make a profit per unit sold at those prices. That's the idea, anyhow.
The PS3, on the other hand, may still be almost the same price as the PS4 when the PS4 launches because the cost of components will remain high for a long time (it has many more components, and many higher cost components than the PS2 ever did). I assume Sony doesn't want to make the mistake of launching this costly of a console again, so they'll probably launch the PS4 at $299 or $399. If it launches at one of those price points, the PS3 will have to be under $199 to continue selling well, especially if the PS4 boasts backward compatibility. I don't know if Sony can manage this price for the PS3 while generating a profit, even after significant cost reduction.