We have to remember the show is 10 years old. Normally, a show this popular only last 10 Seasons, then after that decade, times have changed, the cast has aged, and they always want something fresh to keep people's attention.
I can't say petitions will do any good because your dealing with a company that has to make money to turn a profit for it's investors.
If I recall, the President of CBS saying that as long as the show had a certain amount of viewers, they would leave all three CSI Shows on. With that said, all I can say is that a more effective way to handle this is to write a letter address to the President of CBS.
Don't send emails or electronic petitions. Send real letters. Type them up in MS Word, Sign them, and send them address to the President of CBS. The more "real letters" they get, the more chances are better they will leave these shows alone. That's just my 2 cents worth.