Apple didn't even copy the PowerBook G3... the design was bad for business.
Take my PowerBook (Wallstreet) as an example. It takes almost no time at all for me to replace RAM and the hard drive, I was able to add a second hard drive and replaced the CD-ROM expansion bay module with a CD-RW one. Add on top of that, the processor was a daughter card that could be replaced whole with a newer processor (a G4/500 in my case).
I'll tell you, Apple isn't sitting back looking at someone like me who is still using a system from September 1998 with any form of pride... they are looking at me as lost revenue. Which they should... I would have bought a new system long ago had I not been able to nurse this one as long as I have.
And I think Apple realized this early on with the PowerBook G3s, which was why they took the iBook design in a completely different direction (as in almost completely unserviceable by the end user). That move didn't hurt Apple, in fact it helped. When a hard drive on a iBook or PowerBook G4 isn't large enough any more, people sell their old systems and buy new ones. Why, because it is less expensive (and less trouble) to just move to a new system.
As for the need for a subnotebook... yeah, I sort of see it (though to a degree I think both the 12" PowerBooks and iBooks serve this area).
I was in need for a small computer for school a few months ago. As I'm not rolling in cash, I looked closely at my needs and ended up with a PowerBook Duo 2300c (about $90US for both the PowerBook and maxing out the RAM). With an old system like that I was able to get older (but still very functional) software for my needs (Mathematica for $50US and Theorist for $35US) along with a ton of software I already had.
I have no idea what you guys were thinking a new subnotebook would run, but if I had to buy the current versions of my math software (Mathematica would have run me $1800US and LiveMath, aka Theorist, $300US) it would have been out of the question.
I was thinking about a 2400c, but the fact that I could have installed Rhapsody on it would have been too much of a distraction from my school work. I realized this was a problem with both my PowerBook G3 (Mac OS X) and ThinkPad (Rhapsody/OPENSTEP), which is why I needed a system that could keep me on task... which the 2300c (Mac OS 8.6) does quite nicely.