Limited life span of mechanical systems?

["David Folster" <mr_bond@xxxxxxxxx>]

I have heard from several of the most experienced traders on this list, both
privately and publicly, about how "mechanical systems don't work forever"
and how you have to re-evaluate them as time goes on, etc.  I am definitely
not an expert on this subject, but I would just like to question this point
a bit further.

How do you know when your system has stopped working?

This just doesn't make any sense to me.  What criteria do you use?  Wouldn't
it be too late by the time you figured it out?

Once Mark Brown posted to this list that his best ever bond system in the
world had longest historical DD of 9 months.  So, what then, your system has
stopped working if your DD goes for 10 months?  A year?  What if you decide
to stop trading it and then the system turns around to have a gigantic
runup?  To me, this whole idea seems analagous to trying to trade a
mechanical system while at the same time second guessing it and trying to
cherry pick only the best trades.  You end up taking the losers and missing
the winners, no?

Perhaps the more experienced traders on the list can speak to this question.
Not trying to criticize, only to learn.

Thanks,

David