There's nothing in the release of new models that requires you to upgrade
every cycle. Speaking for myself, I only upgrade when a new model provides a sufficient number of
new goodies at a price insufficiently high as to give me sticker shock (which may in turn mean that I don't upgrade until the new model is not-so-new anymore).
From my point of view it means most bangs for my bucks, from the manufacturers point of view, they only get rewarded from providing significant
real innovations (either new tech or better ways of using existing tech).
Anybody who has a PRS-1 who is seriously contemplating buying a PRS-2 obviously has more money than sense
(and really ought to see me about buying a wonderful, only
slightly used, Brooklyn Bridge
).