One of the “Kedushas Tzion” folks gave an apt parable:
Imagine a Jew whose wife was kidnapped by non-Jews, while the husband was forced to flee. Many years pass by. Finally, the suffering Jew has the grand opportunity to redeem his long-lost mate from bitter captivity (Pidyon Shevuyim) and reunite with her.
But some rabbis absolutely forbid this. “After all,” they explain, “in the many years of separation, your beloved has certainly forgotten the complex laws of Niddah (which involves Kareis)! Halacha would prefer you rather leave her in the paws of her Goyish captor…”