Unfortunately, the sheer dauntingness of it makes it near impossible to get one ready enough to publish. Peoples' groups are just too divergent.
Well, let me clarify: When I say "non-linear, non-railroad", I actually mean "hybrid non-railroad/railroad". Totally non-linear campaigning requires complete improvisation, and almost by definition doesn't suit.
For instance, it is possible to provide a bunch of adventures with hooks and locations which PCs can choose from at will, tie some of them to an overall story arc, and semi-railroad the events leading up to the conclusion of the campaign and accompanying story arc as you tie up loose threads. The result is a campaign with meaningful player choice of what to do and where to go at low and mid levels, and a "foiling the villain" series of railroaded conclusion-to-the-story-arc adventures at high levels. And...I think it can be done in such a way that even the most divergent groups will eventually stumble back towards the story arc plot thread in time for the loose-thread-tying up.
Yes, this can be done with a bunch of adapted little adventures, some of which have been honed to tie in to story arc and setting detail, plus a DM-specially-designed series of story-arc conclusion adventures to make for a campaign climax. I've done this before, minus the story arc and conclusion adventures. Why we don't see it done in cohesive, published form is a bit of a puzzle. I think, perhaps, that designers are intimidated by it, or the publishers don't recognise that models such as the above can exist (or see them as impractical, too high in page count or "pie in the sky"). As the DMG points out, dungeons can effectively be used as a design crutch to limit player options, and the Adventure Path series railroads campaign structure itself in order to retain control.
I suppose that what I'm implying is that railroad adventure series and megadungeons constitute crutches for lazy campaign design...
Hook us all up and make one for us.
Heh. How does the song go? "If you want something done, then do it your-self..."