If you are trading a mechanical system that has been "thoroughly tested" and are encountering a series of unexpected equity drawdowns, I suggest you go back to the system and figure out what's wrong.
If you think you can trade a mechanical system (or any other for that matter without DDs you are dreaming.
But, yes, I agree if the system seems to be 'falling apart' then you may have problems with changing market conditions. My point is you can't assume that your system if failing the first time you have a few consecutive losses.