Not wanting to play the same character any longer and doing different acting jobs IS moving on. Staying on as an executive producer doesn't negate that fact.
He also didn't owe the fans anything. If he felt he had run out of options for Grissom then he was right to step aside. And comparing Grissom to Ray or LF to WP is not fair. Of course they can do a lot with Ray. He's a completely different character who hasn't been explored. Of course LF can do many different things with Ray, his character is new and continually growing. WP felt Grissom's story had run it's course and I think it's admirable that he decided to leave instead of staying on to please other people.
He played the same character for, what, eight seasons? That's a hell of a long time. It really annoys me when supposed "fans" refuse to let go or are quick to criticize someone for making a personal choice for their career or act all butthurt like him moving on was some personal slight.
It's not WP who can't move on, it's some "fans" who feel they have the right to dictate the direction of a show or the course of someone's career.