I think O’Sullevan wasn’t challenged because he was a rather prickly character and no one dared.
I remember an occasion towards the end of his tenure when he kept referring to a horse by the wrong name throughout a race at Goodwood. Someone must have plucked up the courage to tell him eventually because there was an awkward pause before he started using the correct name.
I suppose his commentaries have to be seen within the context of their time. Modern race reading has become more sophisticated and his commentary style wouldn’t be considered up to standard now. But he did produce some memorable calls, such as Red Rum in 1977 and Dawn Run in 1986.