Cezanne is also creating depth by making foreground colors more intense (and the background colors more neutralized), and having greater value contrast in the foreground than the background. It works for me. To my eye, in that second landscape, with the brownish green foliage (forgot the artist's name and can't look back right now), some of the background trees look much too dark. It doesn't convey either a sense of light or a sense of distance nearly as well as Cezanne's does.
As for conveying form with color changes rather than values, I think I read somewhere that you can convey a curve of around 40 degrees solely with changing the color, cooling it as you move away from the light, without darkening the paint at all. Seems right to me.
I see what you mean about moving towards abstraction (although I've never thought of the Impressionists that way before), but if I imagine the same paintings without recognizable objects (say, all the shapes different so that I don't see trees, mountains, fruit, etc.), it seems to me that the paintings would then just be nothing special, of no interest. It seems to me that moving towards abstraction only stays interesting to the extent that you don't actually arrive there! :)
Interestingly enough, the Cubists backed away from completely non-objective art as well. Picasso's Nude Woman below is as far as they got towards non-recognizable objects. Then they came back and developed Synthetic Cubism with flatter forms, brighter colors and recognizable objects.
