why mess with a good design
I'm still quite happy with my PSP 1001, but I see no real reason to complain if they keep refining the system (as long as they don't jack the price up with every iteration)
also, they wouldn't add some radical new functionality to a system this late in the game, like touchscreen. That would just totally fuck up the dynamics of the system. Not only would current userbase would be livid about being "left out", but for a developer to make use of the new functionality, they would have to ignore about 90% (yes, numbers out my ass, yay) of that userbase that doesn't have the functionality, thus reducing their possible profits from a game and therefore the feasibility of producing a game with the new functionality.
in other words, even if it were added, it would never be used