First of all, China and Canada don't "come close" to the geographic size of the United States, they are far larger than the United States. They also have cultures and architectural styles that are as widely varied, if not more varied, than the United States. Very little of Canada looks like the Canadian pavilion, and very little of China looks like the Chinese pavilion, but the whole world "gets it" when they see those respective pavilions.
Furthermore, the American Pavilion is designed to look like Independence Hall, which is a landmark that still exists today and does not, in and of itself require period dress to be relevant or representative of the USA. The only reason you associate the American Pavilion with a specific time period is because of the dress; if the CMs in France dressed like peasants during the French Revolution, you'd associate that pavilion with that time period (although admittedly the Eiffel tower would be out of place, since it wasn't built until 1889). If the Norwegians dressed like Vikings, the same thing would apply.
The United States has many other unique landmarks that are also recognizable around the world, beyond the the aforementioned Independence Hall, including the Statue of Liberty, the Capitol, the Washington Monument, the Gateway Arch, the Golden Gate Bridge, Mount Rushmore, Chrysler Building, etc. Heck, if we wanted something the world associates with the USA, we could build something to evoke the Wild, Wild West. It is as representative of the USA as the Eiffel Tower is of France or a Mayan pyramid is of Mexico or a totem is of Canada.
Personally, I find the period costumes to be a bit contrived and silly, so I have no problem with a change, if in fact one is on the way.