I adore The Guardian, must watch it all again soon. I expect you're hoping for a simple Yes / No answer to your question, but I'm afraid its not that simple 
There had been differences of opinion between CBS and David Hollander about the direction the show should take. Although it got good viewer numbers, consistently winning its timeslot, it wasn't performing particularly well in the 18-49 demographic, which seems to be important in the US because its prized by advertisers (Don't get me started on this, I don't understand the logic of it at all 
if indeed there is any logic. )
DH wasn't prepared to change the show to appeal to a younger audience, and midway through Season 3 CBS announced that it was on the bubble i.e. its future hadn't been decided and was in doubt. I've often wondered if they did this to put pressure on DH to tow the line, but if so it backfired. DH was angry and communications broke down further, to the extent that he didn't attend an event where he would have had an opportunity to fight for the show.
Despite this, I think most people directly involved still felt that because of its good ratings it would get renewed. However, DH wrote Antarctica with a view to providing an acceptable finale if necessary.
Filming had finished and Simon was working on his hiatus movie The Ring2, when he was told about the cancellation, and he signed up for Land of the Dead soon afterwards. To be honest, I don't think Simon was that disappointed about it being cancelled. He has since said that the character of Nick was affecting him and getting him down. He also had a punishing schedule - at an event which he couldn't attend because he was working, Dabney Coleman made the comment "they work him like a dog".
It was certainly a few years before Simon was prepared to consider a TV role again, and when he did return it was in Smith which as an ensemble cast, would have had considerably less pressure of work and responsibility. I think that to sign up for The Mentalist, he must have really loved the script and also had an immediate rapport with Bruno Heller, to feel that working on the show would be an enjoyable experience.
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