I think people mistakenly believe that they would be incapable of falling so far, I assure you that those people are wrong.
No, that's not the point. It has nothing to do with how much he went through or whether it could be enough to break someone that far. It is about Theon not being shown behaving in ways that would make one say, "what he went through has this guy is ready to completely lose all sense of self".
Instead they showed a guy that you'd say, "what he went through has this guy giving in to stop the pain now, but he's still not that close to completely broken".
ETA: If you want an analogy... picture a scene from some movie where a guy runs a half mile or mile. At the end of it he's not breathing particularly hard and has just a bit of perspiration and is standing upright normally. And then he keels over form a heart attack and dies. Then picture the same exact scene except the guy is barely standing, huffing and puffing for breath and sweating buckets, and then he keels over from a heart attack and dies.
It isn't about whether either man went through enough to have a heart attack as they went through the same thing. It's about whether the scene showed the effects on each man to where a heart attack seemed a likely outcome. The first guy falling over dead seems out of place because the scene doesn't show us enough signs that the effects of the run on him are so bad that we feel a heart attack is likely.
Same with Theon. It isn't about whether Theon went through enough he
could break down like that. It's about whether they showed the events affected him to where it was now a likely outcome.
I don't think they did. The moment he gave in just felt incongruous. You could see what they intended, they just failed to show it was affecting him to that degree until all the sudden he's snapped.