How did "Criminal Minds" fare without lead Thomas Gibson? Did it weaken the show?
By the time that Thomas Gibson was fired from the series for his second act of violence, the program had already long since passed its “Sell by Date” and was slowly coming to its organic ending. Gibson’s departure, while unexpected, wouldn’t have mattered as he and co-lead Shemar Moore weren’t on the best of terms anyway, and Moore was already negotiating his exit from the series to star in the police procedural S.W.A.T.share
Criminal Minds was already weakened (in my opinion) by:
The merry-go-round casting of Paget Brewster - They should have simply kept her on the show as her removal and reinstallation and then departure became ridiculous.
The departure and return of A.J. Cook - Again, another bad idea that never should have happened.
The decision to expand Kirsten Vangsness’ role - She’s not a strong performer and she never was. Her limited number fo roles outside of these series is the strongest indicator of that.
The repetitive nature of the stories - There are only so many ways to do the serial killer thing and the series did them all by 2010 or 2011. Everything after that became dull and forced. Things have not grown any better.
The addition of Jennifer Lover Hewitt to the cast - She’s the “kiss of death” to good television. While she’s great showing of her “twin talents” on her own series, she doesn’t play well with others and she’s really not an interesting performer.
So no, questioner, I don't think that Thomas Gibson’s departure from Criminal Minds weakened the show.
It’s been weak and his leaving was simply another sign of its weakness.