If Apple updates the 5 gen software, that will be a first of epic proportions. Upgrade a fundamental shift in software on 2 year old hardware that likely cannot handle what the new iPod OS will/can/might offer? Doubtful.
I have a 3rd gen iPod without any software updates after the 4th gen came out. Some items, like adding "shuffle all" to the main menu could have easily been done but wasn't. I also have a PowerBook physically capable of two finger scrolling, but Apple has and will never release a firmware update for it. I have to use third party software.
Apple wants to sell current iPod owners new iPods. Having a customer base of the iPod does nothing for their bottom line if they don't buy anything else from Apple.
Apple has stated that the iPhone and AppleTV will be different than other products and have more software updates. It remains to be seen if they will do this on a large scale, or continue small updates like adding YouTube video. I don't see any update for previous iPod owners this time around
I think you're right on this after all.... this is the part that I'm the least confident...