I think from a companies standpoint it´s dwindling sales and shiny new toys.
When should it happen?
I for one can say that i applauded the annoncment of 3e, because DMing 2e was a shore with the necessary rules spread out over dozens of supplements. I know i was really giddy at the prospect of a new core with 3 books, with all relevant rules updated and streamlined in a cohesive whole.
That´s dovetail well with my current dislike of 4e: it´s apparently designed to bloat on it´s own books with it "everything is core" philosophy.
So, i think a new edition should come along when the old becomes to convoluted and unwieldy to play
Olli
Normally I would agree with you. Actually I had reached my 3e limit much earlier than my 2e limit...we hardly had any of the 3e class splat books, and I hadn't bought many of the DM supplements. I'd purchased much more 2e stuff, although admittedly that was the college years and I had more throwaway cash.
That said, I've pretty much bought every 4e book out so far, and am looking towards the future releases.
However, I wonder how D&D Insider changes the equation here. First, it allows the easy compilation of all the new crunch that is made available. And in fact, the utility of Insider actually _increases_ as more and more crunch is made available, just as long as the basic framework doesn't change significantly.
In fact, I can see a point 5 years down the line where there is so much stuff in the Insider, when you have a Character Builder that contains all the powers, feats, etc from 5 PHBs and probably over a dozen Power Books...when you have a Monster Builder that contains all the monsters from all the Dungeon, Dragon magazines, 5 Monster Manuals and probably 5-10 Draconomicon style books, plus the other Adventure Tools not yet released, plus hopefully a virtual game table that allows remote gaming...that you actually start making more on Insider subscriptions than the books. But even if you don't, as you add more material to Insider, it does increase the incentive for people to subscribe to it (much like the more people who have fax machines increased the incentive to also have a fax machine).
It could get to the point that a significant change to 5e would be very counter productive...unless 5e was very compatable with 4e...at which point you have to wonder what the point to going to 5e would be in the first place.