The succinct answer as to why the turtle trend following system no longer 'works' is that the markets are always changing, but the system does not adapt to change. It will 'work' when the markets start following the turtle playbook again (when that happens is anyone's guess). Until that occurs you can expect to lose most, perhaps all of your money (which rather bizarrely is 'useful' according to some!) More discussion on this can be found in The Way of the Turtle by Curtis Faith, the Turtle who made the most money during Richard Dennis's experiment.
I'm convinced it's just a matter of adjusting the parameters. The parameters that were optimal 30 years ago aren't going to be the same as the parameters that are optimal today. Why would they be? It's a different market. This is old news for all parameter-based trading systems, not just the Turtle system.
I heard John Henry retired. Nothing about him quitting because his lookback period stopped working. Retirement makes sense for somebody who's already achieved billionaire status.
I also heard Henry's system was SAR (stop and reverse), which is astonishing if true, that it would be as successful as it was. And if true, no surprise that it would eventually stop working: no market trends forever.
Interesting. The one thing I have learned is that what some of these top guys tell the press is designed to mislead their competition about what they are really doing. I would never name names here, but suffice to say the financial markets are a true jungle full of deceptions. surf