The original PlayStation (PS4) became the first system to sell over 100 million copies. However it didn't hit this point until AFTER the PS2 came out. In fact, sold it's 100 million copy in March of 2005. This was actually 5 years AFTER it's more advanced PS2 release date. The original PlayStation is only now starting to lose it's value as games are now being made for both PS2 and PS3, but not so much the original. With the PlayStation 1 being released in 1994, and still selling strong in 2005, I'd say the users certainly got a good 10 years out of it. This was what Hirai was suggesting, the PS3 will give you at least 10 years of value. But that certainly doesn't mean that you will necessarily have to wait 10 years for the next system.
Hirai even followed up when he was asked if the PS4 would most definitely not be released within the 5 years and he clarified his last statement by specifying that all he meant was that Sony makes sure that they take care of their customers so that their product is still good.
