A couple of empirical points. There have been a number of studies suggesting that renewables without nuclear can replace fossil fuels at a cost comparable to fossil fuels. This is not necessarily a "soft path" because the cheapest solar and wind are large solar and wind farms that take advantage of economies of scale. (small isn't always beautiful even with reneewable energy.)
A recent peer reviewed example:
Jacobson, Mark Z. and Mark A. Delucchi. 2010a. "Providing all global energy with wind, water, and solar power, Part I: Technologies, energy resources, quantities and areas of infrastructure,and materials." Energy Policy. doi:10.1016/j.enpol.2010.11.040. www.stanford.edu/group/efmh/jacobson/Articles/I/JDEnPolicyPt1.pdf
Jacobson, Mark Z. and Mark A. Delucchi. 2010b. "Providing all global energy with wind, water, and solar power, Part II: Reliability, system and transmission costs, and Policies. "Energy Policy. doi:10.1016/j.enpol.2010.11.045. www.stanford.edu/group/efmh/jacobson/Articles/I/DJEnPolicyPt2.pdf
Incidentally, Jacobson ranked power sources by social harm in another article.
Ethanol ranked worse than nuclear. Nuclear and CCS coal were ranked equal, so coal without CCS (which is all existng coal minus an experimental station or two) by implication does indeed rank worse than nuclear. Hydro is barely better than nuclear.