For the kind of simple contracts that could be handled by 
  software, people already use form contracts. And remember, 
  you don't need a lawyer at all to draft contracts.

I actually think this market is far bigger than one might think. Legalzoom is doing a $500M IPO and I only heard of them recently. Rocketlawyer is similar.

A high quality, heavily marketed site that had forms for the top 100 or so legal issues (with endorsements from top lawyers stating that they reviewed the contracts) would IMO be quite disruptive. Prenups, startup incorporation, wills, employment agreements, things like that.