https://en.wikipedia.org/wiki/Loving_v._Virginia#For_interracial_marriage

At the national level, 0.4% of marriages were interracial in 1960, 2.0% in 1980,[42] 12% in 2013,[43] and 16% in 2015, almost 50 years after Loving.

The sources cited are Pew Research Center.

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The Census has some problems:

"The Pew Research estimate of the multiracial population takes into account not just how people describe their own race but also how they describe the races of their parents and grandparents. Using this approach, some 6.9% of U.S. adults could be considered multiracial. The Census Bureau counts people as multiracial only if they self-identify as more than one race. According to census estimates, 2.1% of adults were multiracial in 2013."

A lot of Whites already have Indian ancestry. They don't report themselves as bi-racial.

"among biracial adults who are white and American Indian, only 22% say they have a lot in common with American Indians; 61% say they have a lot in common with whites. And only 19% say they have had a lot of contact with their relatives who are American Indian. In addition, biracial adults who are white and American Indian are among the least likely of mixed-race adults to consider themselves multiracial (only 25% do). " https://www.pewresearch.org/fact-tank/2015/06/11/american-indian-and-white-but-not-multiracial/

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I don't think Whites will be insular over the next 175 years and refuse all racemixing. The only reason there are Whites today is because racemixing was taboo for most of the last 175 years, and there wasn't much multiracialism. Mixing will probably get progressively worse and in my observation it is already very bad. Having 128 pure White recent ancestors seems unlikely for future newborns.