There's currently a theory being investigated that most Protestant areas in Europe were actually Orthodox up until quite recent times but when invaded and taken over by Western princes or armies the priests were sacked or they fled and the new rulers simply failed to bring in Roman priests. The populace left to its own started running their own churches.
If you equate Orthodoxy with Byzantine theology or Byzantine praxis. I think that’s hard to prove considering Northern Europe has been profoundly Augustinian since its conversion.
Although if you are referring to the conciliarist movement then you might have a point.
You wrote:
“There’s currently a theory being investigated that most Protestant areas in Europe were actually Orthodox up until quite recent times but when invaded and taken over by Western princes or armies the priests were sacked or they fled and the new rulers simply failed to bring in Roman priests. The populace left to its own started running their own churches.”
Completely false. Take a map of Protestant Europe: England, Iceland, Scotland, Sweden, Holland, and most of the rest had no Eastern “Orthodox” Christians at all - ever - until modern times.