"No fusion" - what nonsense!

Without fusion there would be no Shetland Music, Cajun & Zydeco, Cape Breton Music, Ragtime. In Celtic music there would be no mandolins,banjos ( OK, that would probably be a positive), bouzoukis etc, etc. O'Carolan tunes would be outlawed.

Traditional music positively thrives on fusion. I imagine that there would have been more fusion in the past if economics and invention had allowed more people to travel more easily and further.

The idea of keeping traditions discrete is a spurious affectation of (parts of) the folk revival and these days the "we've always done it this way" brigade.