Like when? The influx of mass immigration began with the Immigration Act of 1965. It's pretty clear by nearly all metrics the country has been getting exponentially worse since around that time.

The immigration people point to as having "built the country" is the Irish and Italian immigration during the industrial revolution. Not only are the immigration rates nothing compared to post '65 but Gaels and Mediterranean are racially similar to the largely Anglo stock of the colonists and early Americans.

There's a massive difference both in proportion and in quality of immigration that directly correlates to our rapid decline. You vastly overestimate the importance of bringing in millions of Mexicans and Indians.