Perhaps in some ways it was a pretty strong worded topic title. In Persia and in the classical world, the respective monotheistic religions did preserve much of what went before. However, in Northern Europe including Britain, the religion which was displaced was so successfully propagandised (sp?) against that it is perceived to this day as satanism or witchcraft, and evil in nature. (given that Satan is a construct of the Monotheistic world, I do not regard Satanists as being in any way part of the Pagan religion, even though they themselves may latch on to it. In my view, a Satanist is simply a renegade Christian).
In Ireland, the most comprehensively Celtic country in the modern world, the Catholicism of the Romans totally has supplanted the original Celtic religion. The thing that makes Ireland interesting, however, is that the conversion was not brought about by invaders - it seemed to be accepted willingly even though only introduced by a handful of evangelists. Either the Irish people genuinely found this religion attractive or the Irish kings considered it politic to convert, as it (belatedly) admitted them, in a partial sense, to the Roman world. I think the second explanation is most likely, as Christianity even in the 5th century was rapidly assuming its repressive, mediaeval guise.