Wait a second. Your mixing up all kinds of things here. You are talking about the Christianization of Europe which relied on adopting pagan elements to make Christianity more appealing (it also relied other things as well like force). That is way more complicated than borrowing headgear and slacks. Minority cultures die out because they get displaced, people take their land, they are the victims of genocide, they are pressured to assimilate, etc. I don't think people appreciate the kind of conspiracy minded thinking going into this narrative you are building. If you examine the history of the Christianization of Europe, I don't think this is the lesson you take from it. I would say more often than not, an unwillingness to engage with outside cultures, rather than a willingness to engage, is to blame for bloodshed, genocide and cultural destruction. You are cherry picking aspects of history to make a case for people not borrowing musical scales or visual patterns that come from cultures outside their own.
Also very important here: without that kind of approbation we wouldn't have our own vibrant culture that includes things like celebrating halloween, decorating Christmas trees, etc. Everything comes from somewhere. The pagans got those ideas from other people too. This drive to distill cutlural elements to some kind of mythical pure form, I think is deeply misguided.