The (relatively) recent release of 3.5 after 3.0 is definitely the source of the current speculation, even if there was speculation back in the day, too. The reason is, 3.5 supported everyone's best and worst notions about D&D.
If you were cynical and jaded, then 3.5 was your worst fear realized. WOTC was a soulless corporation sucking hard-earned cash from its helpless customers, and 3.5 proved it. And since they got away with it then, there's no reason they won't try it again every few years, and we're now at four years for 3.5. We're looking at a dark dismal future of endless edition progression until we throw off the shackles of Hasbro and live in a utopia where everyone plays (INSERT FAVORITE EDITION HERE).
If you were optimistic and idealistic, then 3.5 was fantastic! Sure, it didn't fix everything, but it made a lot of stuff better. It wasn't that hard switching over, considering how much better your game plays now. If they improved the system that much in three years, how much BETTER will it be now? We're looking at a bright future of continually improved mechanics, making our favorite game better than ever!
So, the people who hated 3.5 fear 4.0, and the people who loved 3.5 anticipate 4.0. Or, sometimes, the people who hated 3.5 anticipate 4.0 and the people who loved 3.5 fear 4.0. Either way, 3.5 so polarized everyone's reaction to a new edition that it's now a part of how we talk about D&D, possibly forever.