[popo]

I just don't see the positive side of "lenses becoming outdated" 

I guess the implication is if an older lens is considered outdated, there must be something better available. And if you don't need that new performance, the old one will generally cost less than it did when current.
 

My Canon 70-200/2.8 IS (non-II) is seeing the third camera body (and double that if I count the stuff loaned at one point or another). Though I'm pretty sure I'm done upgrading bodies now.  Even more wide-angles have come and gone during that time.

Similar here... my 100-400L has been my main lens on the 50D, 7D, 7D mk2. While there is the shiny new mk2 out, I don't see enough reason to get it until the old lens dies, or something else much better comes along. Body wise, there was a noticeable jump from 50D to 7D (both AF and sensor), less so to the mk 2. So in a few years whatever the mk3 is better be really special.

In general, I still have the "good enough" theory on buying. If performance isn't good enough, you will spend more to get the best available. Once it reaches a good enough level, you stop there and other factors start becoming more important. e.g. possibly smaller size of mirrorless, or you buy lower down than you might have previously as it will still do the job.