Thomas Gibson had a history of inappropriate behavior. No one gets fired for just one offense. Since his firing, many people have come forward to say that he had a temper and behaved inappropriately. For this reason, the audience understands that he can't be on the show any more, and the writers will do a good job of developing the script without him.
When Gibson was fired from Criminal Minds, the show lost a character who provided a tough exterior with internal struggles, which provided more depth and interest to the show's plot. The analytical mind of his character played well off of Rossi's character and brought a more objective view to the cast.
The show will not be as good as before. Having fired Thomas Gibson from the Criminal Minds Show, will have the show boring and will also loose some viewers and followers. Simply because, some shows are just made for particular people who fit-in and no one can fill that bracket. This may result to loose of fans and a lower rating too.
Gibson was the heart and soul of Criminal Minds. I know it was an ensemble cast and people have left the show before, but Gibson's character arc and his acting just really made the show what it is. I'm surprised that he was fired, I didn't know that popular characters could get fired from a show that's on air.