This isn't a movie, Master. Christ almighty
I think you're the one who has this confused with a movie. Did you watch Balboa and think, "Hmm, I bet a really old guy past his prime could do this in real life too."
And let me add that if this was Shaq in his prime, I'd agree that he wins a majority. Because he freak athlete trained body would give him a tremendous power advantage. And his stamina would not be as great of an issue. He'd be seven feet of muscle.
But in this scenario, you seem to think that older retired Shaq could just waltz right into the ring/octagon at a moment's notice and win a majority against a highly skilled fighter. Being a big guy counts for a lot in a fight, but not this much.