American here and I do hold a few thousand in both Euros and Pounds, which is why this subject interests me. I do see the difference between holding a few for when credit cards don’t work or you just need cash and those like me that hold more. Just a few days ago a German restaurant didn’t accept cards, something I didn’t think to ask when sitting down.

I just like having physical notes and it gives me an extra feeling of security.

About the USD, I’ve read that the original plan was to make the old versions obsolete but that there were so many 100 dollar bills around the world hidden under beds that it wasn’t practical and would have caused great chaos.