The problem is assuming we wouldn't be using Hammer... "Hammer" is simply an evolution of Valve Hammer Editor which is an evolution of Worldcraft, which was actually designed for making Quake maps before the creator was hired by Valve and it was turned into a Goldsrc editor and they built Half Life using it.
I'm sure whatever we have will simply be the next step in the progression of the editor, just like Goldsrc was slowly morphed into Source, which is slowly morphing into "Source 2"... Valve does not have a history of starting things fresh and new, the old just gets changed. Even during the lifespan of Source it has been altered and upgraded several times with features that don't work on older versions. Heck, even Goldsrc itself was not a scratch engine, but a modification of it's predecessor.