In what other major sport are there so many stars that aren't American? My point is very much valid whether you buy it or not. If all the Latino stars were American, you would see a lot more baseball players doing commericials in the states.
Assuming I buy into your premise that foreigners are not valuable marketing assets, here are some MLB stars that are American off the top of my head...Harper, Trout, Verlander, Kershaw, Votto, Arenado, Stanton, Judge, Bryant, Betts, Bumgartner, Scherzer, DeGrom....