Remathilis
Legend
There are no expiry dates on game systems, to me. I still play OD&D and AD&D. The notion that a design would become "outdated" is something that really makes me scratch my head in disbelief.
The only incentive I see to have a new edition of a game is to basically re-sell core books to its core audience, since core books are the only real sellers in an RPG line of products. The rest is just marketing speech destined to sell the idea of buying the same stuff all over again, as far as I'm concerned.
Game design evolves. For example, I don't see too many people who think that changing the Save System from Five Arbitrary Categories (Wands, Dragon Breath, Death Magic) to Three saves based on type of resistance (Fort, Ref, Will). Unless your specifically going for the nostalgia factor, I don't see a point to making a d20-based game that emulates the old save categories again.
More to the point; I see Pathfinder eventually tightening up and better integrating certain design elements that will make former material obsolete. Reducing the number of status conditions, speeding up combat resolution (a continuation of the work done by CMB/CMD), revamping how magic items are designed, or simplification of monster creation/stats are all things PF could continue on and, by removing the vestiges of backwards compatibility, truly innovate on in a way different from how WotC chose to do so (and perhaps stay a little more "true" to the source material than WotC did).
While anyone can play (and enjoy) older games for their own merits, I seriously doubt you could sell an AD&D 1e game in a modern gaming store today*; games and gaming have evolved from that point.
* Ignoring, for the moment, the OSR, which does base part of its appeal on mimicking the Older style of gaming. A brand new game with AD&D's level of development would never survive, as evidence by the sheer number of fantasy Heartbreakers out there, even by Gygax himself...