Well for one there is no reason to assume it will in fact be decompressed. The books might be very comprehensive.
And "especially when mechanics and power creep render the earlier career books less useful then the later ones" seems pushing it a bit as well.
By decompressed I mean the 6 career books would be spaced out over a long period of time, not necessarily the material in them would be less dense. Longer periods of time mean more things can happen that disrupt the process - including things outside the company's control.
And creep/change is just a thing that happens in game lines - it isn't necessarily a bad thing, but it does mean books made before the new thing (be it a piece of gear, or a talent or just a rules interpretation change - like the one about weapon qualities requiring a successful (damaging) hit before activating, for example) are less useful/desireable than books made after the new thing. Another reason I prefer shorter, more planned out game lines.
I've just seen games really hit rough patches or crumble altogether when books are spaced too far apart or are too specialized. In particular I'm thinking of the problems White Wolf, Wizards of the Coast, Margaret Weiss Productions, and Eden Studios had - the latter three in particular with licensed properties.
It sounds like I am being negative, but I just don't want to risk bad things happening to a game I am really enjoying. My ideal - each line (Edge, Age and Destiny) gets 4-5 line-focused books and then they are done and if there is a second wave it's either a cleaned up edition change or it's entirely optional all-in-one stuff for all three lines (era books, gear stuff, etc).