Pretty much everyone is expendable on SPN except Sam and Dean. However, there are major differences between Bobby and other supporting characters. Other recurring and regular characters are tied to their stories. When their stories end, there's no longer any need for the characters.

But Bobby isn't tied to a story. He's intricately linked to Sam and Dean.

Bobby knows Sam and Dean better than anyone. In some ways, he probably knows each of them better than he knows himself, better than they know one another, and better than the audience knows them. He's been their "uncle" since they were young...