As someone who travels from the US to Europe a lot, my reaction would be sadly shrug my shoulders and not travel to the EU any longer if the requirements were a

As someone who travels from the US to Europe a lot, my reaction would be sadly shrug my shoulders and not travel to the EU any longer if the requirements were as onerous as, say, China or Russia. (Letters from an organization, etc.) Yes, the UK put ETA in place but that's trivial and mostly symbolic. Took me about 2 minutes to complete which was mostly about grabbing my passport to get the number.