To be fair, he hasn't been in the position to make a Sunday comeback more than 5 times. He's usually either running away with the trophy already or is more than 5 shots back. Those times he has been within striking distance, he hasn't got it done, but he has never lost ground. Most of the time, it comes down to whoever is in the lead plays a great round of golf on Sunday and he can't make it up.
I agree that a comeback win in a Major will cap off his career, but the dude is already the best without it.
If he retired today would you say he is the best? I just get the feeling that people put him as the best all time because of what he will probably do. Everyone think he will break all the major (events as well as in terms of big) records but he hasn't yet.
Also on the mental toughness thing, it's not that hard to distract a golf player and break his concentration. We have seen this from many players including Tiger.