I guess the analogy is that FS9 is to FSX what Windows XP is to Windows Vista.
i.e. why fix something that ain't broke?
I've been flight simming since the early 90's and before that I "simmed" (if you can call it that) on a Commodore Amiga. FS9, in my opinion, is the only FS version that is/was not "wanting" for an update. Sure FSX adds some new features but novelties wear off quick in this field and what matters to serious sim pilots is the quality of the realism and authenticity and the options for quality add-on's.
Eventually market forces will push FS9 out of date as we invest in newer, more powerful PC's and when enough add-on's are produced for FSX only.
Otherwise FS9 is the currently the best. Eventually, though, in time we'll be able to do more with FSX, or at least we should be able to if it has been designed for the test of time. FS9 is a hard act to follow.