Well even in its historical phase it was still pretty similar to what followed. The show's never radically changed (aside from as mentioned for the third Doctor) I mean compare that to something like Fringe that began as an X-Files style show, morphed into one about parallel realities more like Sliders before ending up as a near future dystopian alien invasion/rebellion story like V! Who has never had that kind of radical overhaul (and I'm not saying it should and it might be the end if it did, but that's the point, its longevity, like Bond's, is based upon a mixture of familiarity and adaptability)
I don't agree that
Fringe's changes are more radical. I'd say
Who's change from straight history serials with no sci-fi content beyond the TARDIS crew themselves to pure sci-fi where all the historical stories had aliens and Atlantis and such in them was similarly major, as was the 3-year near-abandonment of space-time travel in favor of
Quatermass-style adventures on near-future Earth.
Besides, there are more ways a show can change than just in the superficial concept, which is the point I've been trying to get across. Changes in style, emphasis, and sophistication can be just as important. Certainly Russell T. Davies brought a whole new approach to the way the show and characters were written, more grounded and dramatic, although some of the seeds of that were laid earlier with the Seventh Doctor and Ace. And as I've said, Moffat's turned it into a very different show that's less about the Doctor exploring the universe and more about the universe reacting to and revolving around the Doctor. I'd say that's profoundly different from a structural and thematic standpoint, even if the superficial trappings are the same.
Fringe may have shifted its superficial emphasis in terms of what the characters were dealing with, but its approach to the characters and the tone and style of the show remained pretty consistent, which helped it feel unified despite its regular reinvention of itself. And whether we knew it at the time or not, the entire series was essentially driven by Walter Bishop's past actions and their consequences in the present and future. So basically the whole thing was like Moffat's
Who, in that essentially everything that happens is an outgrowth of the past actions and choices of the mad genius at the heart of the show.