I suppose no one wants to reignite "what is a traditional instrument", and wisely too.

You need to cover all the influences: ancient roots (traditional bagpipes say); fashion/art music influences (availability of instruments like fiddle, flageolet, flute, guitar; the Frenchification of the Northumbrian and Uilleann pipes); popular music influences (early/ late music hall and theatre bringing concertinas, accordeons, melodeons, guitars); communal culture like brass bands.

And then discuss why only some of those inputs took in the "folk tradition". If it was the availability of cheap instruments, what happened to all those redundant art music lutes after the mid 18th century? Church cornetts and serpents? Why did the concertina become a "folk" instrument but not the guitar (in England) until the American influence of the Revival(s)? Why were the smaller brass instruments not used in, say, Morris? (or the larger ones for that matter) Or were they? Why did bagpipes go out of fashion? I can tell you why the saxophone and oboe haven't caught on in Irish sessions: the one's too loud and strident, the other too hard to play.

It's a deep subject, that touches on late unhappy exchanges here, and though I'm no expert, it seems solid information is either hard to come by, or discounted for ideological reasons. Who played what, and when, and was it traditional music or some other kind?

I suppose you could start by talking of self- defining musical communities, and how they define themselves, and go from there.