First off, let’s discuss why they should keep him. For one, he’s arguably the best pitcher this franchise has had in decades. His 2014 post season performance might be one of the greatest of all-time.
Bumgarner’s only real injury history are injuries that didn’t inhibit his ability to pitch, and none of them have much to do with ligament structures or damage.
There is also the factor that they likely don’t have anyone close to replicating his output within the organization. To be fair, none of the guys drafted the last two years look like they could turn into Bumgarners, and neither does any of the guys currently in the crop of their most MLB ready prospects, including those already there.
So then, outside of Bumgarner next season, is there any free agent who could replicate what Bumgarner has done? Probably not.
There are still some solid options that will be around Bumgarner’s age and ability, but Gerritt Cole and Chris Sale seem like the only two who could really come close to providing the same “Ace” bump that Bumgarner brings, and Sale is the only one with a real track record of postseason success, thanks to this current run with the Red Sox in 2018.
So if they couldn’t replicate Bumgarner with anything they have in house, or it would be incredibly difficult to find something similar on the free agent market in the next year or two, why trade him, especially if you are intent on competing? You don’t.
What happens if you intend on competing, believe you can compete, but fall short just like 2018? Then the question becomes, do you believe you should sign him long term after 2019? Of course, that’s another point for discussion.