Well, WotC talked a good game about how 3.5 was backwards compatible, too. A lot of people are still mad about getting burned on that one. Rightfully so. Right now it's all hype and PR. We won't know until the books are printed and in stores.
But yeah, they're definitely trying to phase out the term edition. Doubt that will stick. When the numbers go down again, they'll be real quick to reverse course and start hyping 6th Edition or whatever they'll call it. I don't think they're going to pull of the "no more editions" thing. There will come a point when they'll have too much cruft and need to rework things again. No system is perfect so it's not going to last forever.