I am a big Family Guy fan and I don't agree with this at all. You can't just kill off a major character after 12 years. Brian was part of the fabric of the show, one of the reasons I love Family Guy was the dynamic relationship between Stewie and Brian, lots of funny storylines and gags were developed from this.
I think they will eventually cave in to viewer pressure and backlash and bring him back, like how South Park bought back Kenny after killing him off "permanently".