The real question is whether the new structure of the game and the new focus on long term, sustainable business (subscriptions, increasingly modular game, planned editions instead of waiting for crisis points like the 2e/3e changeover) will allow WotC to stretch out the non-bloat portion of a game's lifespan long enough to then switch to 5e without feeling rushed.
And of course that gets to an entirely new issue... the feeling that an edition change is "too early" is, in a certain sense, an argument that there wasn't enough bloat yet, that more bloat would have been appreciated, because then, seeing the damage caused by the bloat more clearly, the fan base would be more accepting of an edition change. I have no idea what to say about that.
Perhaps WotC's 4E business model is to create enough bloat fast enough themselves, such that a 5E D&D is viable and more palatable within 4 or 5 years. They may be looking at the 4 to 5 year lifespan of 3.5E from mid-2003 to early 2008, as an optimal edition lifetime for 4E.
At the same time, they can't release too many books every month. They don't want to risk what happened with the glut back in the 2E AD&D days of too many settings and too many books. They probably also still have fresh in their minds, the glut of Forgotten Realms and Eberron books during 3.5E's lifespan which may not have been good sellers. I wouldn't be surprised if many of the 3.5E Forgotten Realms and Eberron books did not even sell out their first printings. I've seen several 3.5E Forgotten Realms and Eberron splatbooks in the discount book section at several "big box" bookstores for $6 a pop.
With the economy presently in the toilet and possibly for the next year or so, perhaps the one new book released per month strategy is the only one which is presently sustainable. All the extra stuff is put onto DDI, instead of books. If some of the extra stuff on DDI turns out to be promising, they'll eventually make their way into the yearly Dragon Magazine Annuals books they have planned or even possible future splatbooks.
WotC probably still has the release of the 3.5E core books and subsequent fallout still fresh in their minds, and don't want an obvious repeat of it with a 4.5E. In hindsight, releasing 3.5E just three years after 3E was probably just too soon. There wasn't much of an appetite for a new edition, just after three years.
From looking at the catalog of 3E splatbooks released by WotC before mid-2003, there didn't appear to be any major bloat issues happening at that time in their own books. Most of the 3E "bloat" was happening in the 3pp market. Perhaps the release of the 3.5E core books in mid-2003 was WotC's way of slamming the brakes on the 3E "bloat" in the 3pp market, at the expense of angering D&D players and the subsequent fallout. WotC wanted to regain market control of the "bloat" on their own terms, and hence all those "Complete ...", "Races ...", etc ... splatbooks.
I wouldn't be surprised if the Hasbro/WotC executives and bean counters at the time (2001-2002), were paranoid of completely losing control of their own D&D product and markets. They had paid a lot for the acquisition of WotC, and didn't want their investment to fall into the toilet from a hippie-like "open gaming" concept.