You are indeed correct -- a sub .300 OBP is awful.
Let's be glad we're not talking about Middlebrooks.
If we use the logic of career, as you do, he has been under .300 exactly twice: in Lowell, in his first season in pro ball, and in 2013 MLB, when he was both hurt and confused, in his first "full" season of MLB.
Let's not go calling him Kingman or Rob Deer. He's not even close to that kind of player, or hitter. We can disagree whether he will "ever" significantly improve his approach. If you go by his career, he doesn't need "significant" improvement. He needs progression, sure, but if he progresses according to his most similar comp at his age, he progresses like Frank Thomas.
Now that's B-Ref talking, and that seems quite optimistic (especially since one of the other comps on there is Mark Reynolds). But I only bring it out to say that the Kingman comp is simply not even an approximation.