My system has been working for 4 years and almost the 20 years I back tested it I've had to adjust it for volatility,in 2005 I had 20 point stops and a 30 point profit target,Today I use a 40 point stop and a 60 point profit target.The system is the same though
You need to be constantly evaluating different markets and tweaking your systems if required. A market might dryup in liquidity or its bid ask spread might double or half and it might not be tradeable for your system anymore. If a person is trading a basket of markets that basket is likely to change over the years and new better markets emerge and older ones die off. If a person is trading only one market then that market is subject to changes by the exchange or regulations. And you might have to tweak or even abandon your system completely. Even if you think your system is static, the floor is constantly shifting underneath you and you need to constantly adapt.
I like this answer.. For the guys that are saying their system still works, are you using indicators or just price action..? I think that's important to know..
My long-term investment system has been unchanged since 1998 and continues to do well. The short term aggressive day-trading systems have much shorter shelf life. If you rely on specific market micro structure (e.g. Flash orders) those systems can last as little as months before the inefficiency is found by others and the game gets harder