Morris dancing is a mostly English tradition of highly stylized folk dance. It is definitely old -- Shakespeare's Dauphin mockingly compares the English preparations for war to a Whitsun morris dance. But like most things in the English folk tradition, it is not as ancient as we like to pretend: it goes back centuries, not millennia. I think I am correct in saying that without exception, present-day Morris sides all go back only to the Victorian folk revival; there are no places where there is a continuous tradition going back to the fifteenth century. Almost certainly it wasn't an ancient pre-Christian fertility dance, but it's quite fun to pretend that it was. Everyone involved seems to agree that on one level its quite silly: beery men with bells on their fingers and toes and waving hankies in the air -- but its also colourful and fun and almost always involves good tunes. The idea of a lot of groups of people taking a lot of trouble to keep up a tradition which is on the surface a bit ridiculous seems a properly English thing to be doing.
Every few years someone in the unfolkie media spots that a few Morris sides perform with black make-up on their faces.
I am not sure whether anyone is really (as opposed to theoretically) offended by the sight of fat white people with boot polish on their faces waving handkerchiefs in the air. (I thought that all the most important philosophers of the age were agreed that there was no such thing as giving offence or if there was it didn't matter?) But I am completely certain that no serious harm is done to the Tradition if the flanneled fools leave the boot polish off. I didn't see a single black face side at Sidmouth; I think all the Border groups have taken to painting their faces red or blue or green. Which definitely offends no-one and is actually more fun.
The etymological fallacy is just as much a fallacy when applied to folk traditions as when applied to -- well -- etymology. A word means what you mean by the word, and what other people understand you to mean by the word -- not what Simon Heffer says the word "originally" meant. Grammar nerds may or may not be correct in saying that at one time decimate meant "to reduce by one tenth": but right now it means "to lay waste to" because that is how people use it. They are both wrong and offensive when they claim that wog is not "really" a racial slur because it "originally" meant Worshipful Oriental Gentleman.
I don't think that it follows that you can merely add the suffix -up to the name of a particular group and take that as incontrovertible proof that no-one outside that group can represent a member of that group on stage or screen. I don't know if Christians can ever properly understand what it is to be Jewish. Probably they can't. I don't know if Jews can convincingly play Christians. (I might be inclined, like Laurence Olivier, to ask "if they have ever considered acting, darling) But I am pretty certain that it is not helpful to accuse Kenneth Brannagh of "Danishing-up" or "wearing Dane-face" to play Hamlet.