Well there is DNA evidence now that settles this controversy. The Ashkenazi Jews originated in the Caucuses of Southern Russia and have no genetic relationship to the ancient Hebrews. Furthermore, there is a criterion in Jewry that to be a Jew one must have a Jewish mother. Because there is a genetic requirement to establish Judaism then raising the issue of their heredity is fair and in light of the conflict between Israel and Palestine and the so called "Jews" claim to the land based on heritage it is a relevant topic.
Using your logic the black Americans should have changed color a long time ago. Maybe a white man can go to Africa and use your argument to get citizenship in Nigeria.