Elizabeth,
And what good exactly came from people focusing on diabolical possession?
I am not suggesting there weren't a few conversions, but Medjugore produced some too.
The point is that diabolical possession has always existed, and has never had the primary interest of Christians. It's always been the concern of a few specialists, at least during healthy periods. The unhealthy periods were when the Protestant witch-trial mania infected Catholic areas to some degree.
What excited people about Martin's book was the sensationalism of it, the prurient interest it fed, the sense of a sporting contest that he created, in which the Exorcist was risking his own soul to come to the aid of the poor innocent who was possessed. Those are all worldly motives. (And as I've said, the risk to the soul of the Exorcist is fairy-tale stuff anyway.)
What was the practical benefit? Nothing.
In 1975, when that book appeared, the crisis in the Church was as dramatic as it ever got, because floods of priests and religious were still apostatising, clown "masses" and such like were proliferating, etc. Martin noticed none of it, didn't care. He had more exciting things on his mind, especially making money and presenting his New Age religion of Immanent Jesusism.
Martin was a dangerous distractor, not a shepherd.
We don't need to worry that we might become possessed. We need to worry that we might not guard our tongues, our eyes, our thoughts, and that we might offend Almighty God. To avoid that catastrophe we need His graces, which means we need to live our faith energetically, generously, selflessly.