Agreed, but at the local level and, outside of big bruisers like CA and NY, the state level, you can do more with less money, and with less outside interference. And Sanders was always an opposition-oriented far left voice in Congress. A centrist independent with a track record of success as governor in a populous state would have an easier time making national inroads than a fringe-y independent like Sanders. You're right, though, about either party's built in immune system making it hard for any outsider to get a fair shake.