It's a reasonable seeming law, yes, just as the categorical imperative is - but what if your culture says that the more skulls you collect, the better a person you are? It's easy to define a law in isolation from social pressures which defy that law. Take the inquisition - by burning hereticks you saved their souls, and this was therefore good - so do as you would be done by applies, even if it comprises an auto da fé.
With re: apathy - "all that is necessary for evil to triumph is that good men do nothing" - sure, but this still relies on a fixed idea of good and evil, and that you can recognise "evil" at its inception. The issue there is that tragic consequences tend to only become apparent in hindsight.
I do personally agree with "do as you would be done by", but you can't apply it as a universal law without the complete homogenisation of global culture - and even then folks will develop their own ideas of what constitutes good and evil.
Ultimately, it's all about societal consensus, which these days (since bernays in particular) is media led.