I think all one needs to do in order to assess whether or not you can change the system from within the system is look at modern US politics. We recently came to the brink of financial collapse at the behest of a small faction of uber-conservative tea party republicans. They're a new political contingent of US politics that didn't exist only a few years ago. They voted and now they have major sway within the system. If the Left were as organized as the right they'd get a lot more done. The OWS movement really wasted some opportunities IMO.
I tried to find ways to mobilize and organize some of my friends in OWS Oakland and SF when I lived in the Bay Area because I had a good number of friends out there taking part, but it just wasn't the right movement for it, seemingly. The people all had a cause, and maybe tangential causes from there, but it was exceedingly broad and fractional. They were all centered around making the financial sector responsible, but that's moving into territory that hasn't been done outside Glass-Steagall. There are so many ways to approach it. The major problem progressives and the left face in mobilizing is that it involves moving forward and doing new things to change, while the right has it easy due to simply moving backwards. Their ideas have already existed for a long time and have been implemented, the left attempts to progress from them.