Answer #1:
Jack Schwager: "You mentioned that you've shared your trading methods with other traders. Aren't you concerned that revealing your approach could destroy its effectiveness as other people start to use it?"
Linda Bradford Raschke: "I truly feel that I could give away all my secrets and it wouldn't make any difference. Most people can't control their emotions or follow a system. Also, most traders wouldn't follow my system, even if I gave them step-by-step instructions, because my approach wouldn't feel right to them. They wouldn't have the same confidence or comfort in the trading method as I do. But for argument's sake, let's say that showing my methods to other traders did eventually cause some of the patterns that I follow to change. If these patterns changed, new ones would be created, and I'm confident that I would find them."
--From The New Market Wizards
Answer #2:
Plenty of professional, extremely successful traders do reveal their profitable strategies and continue to do so. For example, Dan Zanger's nightly newsletter shows charts with his picks, entry and exit points for the next day, along with explanations of how he arrived at those decisions. Trade the Markets does the same in their nightly newsletter with swing trades, and they post the last several years of their picks so you could backtest them. As others have mentioned, the Woodies strategies have been public for years.
I have seen no evidence of strategies that were ruined by being made public (though it might exist). But clearly there is evidence of strategies that survived being made public.