This is anecdotal as well, but when I've spoken to Europeans about coming to America, five places keep coming up to the exclusion of almost all else (in descending order): New York, California, Miami (or Disneyworld, if they're Germans), Vegas, and various national parks out west. I could be totally wrong, but it seems like the grand tour of the Eastern Seaboard is something Americans do. Europeans really don't care about the three-cornered hat stuff. And if they want monuments and history, then coming here for that would be like us going to Copenhagen for soul food.
You might be right about this summer, and I think the Barnes has been vindicated as a genius move. But to sell this city to more Europeans, we have to answer this question: If I only have a few days in the whole country, why should I take time away from New York and spend it in Philadelphia? Philadelphians are spectacularly bad at answering this.