It would take 100 years for a single demographic if they were using all the 65000 annual H1B visas to reach 2% of the population assuming the population doesn’t grow for a century.
First, are we talking about the green card or H1B? There is no quota on H1B and we already see a shift in culture and demographic in many companies.
Second, an immigrant can take their spouse, have kids, sponsor their families' visas, etc. Any one of these will double their demographic's representation, and they are not mutually exclusive, i.e. it can easily be triple or quadruple, or more.
This argument makes no sense at all.