The majority of prisoners are in for petty crimes anyway. Might as well just fine them instead of forking out to keep them in prison. I think prison should be exclusively for people who are a danger to the public, and those who commit massive crimes like fraud and stuff. I think I'd include benefit fraud in that too though, as fining probably isn't the best option for those who are already on benefits to survive
I don't think this is that bad of a proposal, could potentially sort the whole thing out, or as most of you feel, it may fall on its backside.