Culture - I mean at its core the US culture is an immigrant culture - one ultimately ever evolving shaped by successive waves of immigration since its founding but underpinned by an idea - all men are created equal, individual liberty etc. etc.
The US 'culture', the US nation - the thing that makes it different from every other nation state or kingdom that's practically ever existed ever - is that its based not on tribalism ( which is a derivative of DNA/ethnicity) or based on some religious fanaticism - but rather its based on an idea......the idea of individual liberty, freedom & the pursuit of happiness
What makes America dynamic.... is its very lack of reverence for the past...for old cultures, stale ideas...its unwillingness to "keep" a static US culture....America's ability to reinvent itself is its secret sauce I would posit....and that re-invention is no small part driven by the immigrant culture and the drive to reinvent oneself that immigrants have in abundance....to drive forward, to do things differently, boldy, take risks and ignore the old ways of doing things.
Tinkering with this uniquencess IMO is akin to Coke suggesting they should tweak the recipe.....immigration is the USA's secret recipe.