By definition these 10,000 founders wouldn't be taking jobs from Americans: it could be part of the terms of the visa that they couldn't work for existing companies, only new ones they'd founded. In fact they'd cause there to be more jobs for Americans, because the companies they started would hire more employees as they grew. ----- Of course they will not be hired by other companies...Legally...But USA is not the country where illegal employment is punished.
Every time when you open access to your (rich) country to poorer people think about how the system can be abused.