doctor who - Did the pure Daleks survive?

You can't judge a Dalek by its casing. The episode "Dalek" establishes this style of casing as being the style used in the Time War - when the Daleks were still pure. Davros was "lost" in the first year of the Time War, as stated repeatedly by the Doctor, most notably in "Journeys End" where the impure Daleks are introduced and their origin explained. Davros was brought out of the Time Lock by Caan, and using his own body cells (showing exposed bones on his chest as he explains), he grew a new breed of Daleks. These impure Daleks have casings that are largely modeled off of Caan and the other war Daleks.

The remnants of these impure Daleks find the progenitor device created during the Time War. It contains pure Dalek DNA which is then used to create the new "pure" Daleks in "Victory of the Daleks" - and the casing colors are stated to designate the function of each Dalek. The restored "pure" Daleks go off and repopulate with the knowledge that their worst enemy from the Time War is still alive.

That is as far as we know for certain. Extrapolating from that, though, the decision to return to the war model casing makes sense.

There are also "millions" of pure Daleks with these war style casings who were trapped in the Void in "Parting of the Ways". In "The Next Doctor", it is stated that the Cybermen stole the device the Daleks were working on to escape. This second possibility would explain the mixture of new white, blue and red Daleks among the brass colored Daleks. It also explains the sudden appearance of a parliament in what had always been a hierarchical command structure - two groups of Daleks with their own hierarchy and needing to integrate might combine their command structures in a new way.