Tony Vargas
Legend
How long an ed goes is more a matter of marketing and corporate goals than design. 2e and 3.5 were tottering under a surfeit of supplements years before they ended. 4e is still mechanically viable, in spite of Essentials. It's business factors that are forcing the rev-roll. WotC over-promised with 4e, and are doing so again, with 5e.3.5 was annoying, and the entirety of third edition lasted, what, seven years? (~2001-2008). 4E at four years? I hope part of the design is building something that will last.
5e may very well be the last edition of D&D, at least for a while, when it fails to meet the impossible goals its been set and Hasbro shelves the line.
d20, OTOH, thanks to the open-gaming strategy, will always be there...