Well, sure... and they obviously take advantage and benefit greatly from the fact that we're so nostalgic about Disneyland. But there's also a ton of other stuff we're nostalgic for that they can (and do) integrate into the parks since Americana was all the rage.
Point being... Disneyland was not designed to be about Americana. Disneyland was designed to be about what people were nostalgic about in 1955 (yes, the very same year that Marty McFly travels back in time), which happened to be Americana at that time.
The fact that so much of the park has remained rooted there, IMO, can be attributed to many things -- not the least of which is the timing of Walt's death, the near-immediate shift in focus to Florida shortly after, and the uncertainty of the company for many years during that time all of which lead to a pretty significant halt in DL development for years to come.