He's the dog of all dogs. What real dogs look up to. He's smart, quirky and an integral part of the cast.  You can't just kill off the pup we've learned to love.  Even though he drives a prius... What you should have done is crash the prius and get him a real car. 

Stewie needs to go back in time and save Brian from getting hit.  

Plus, he never finished his novel.