What about the continuing evoloution of each "race"?
White european americans have changed a lot in just the past 100 years.
Look at some of those old suits of armor. Could an average person fit in one now?
in time all will blend in genetics and at that time people will start being prejudiced based on nose or ear size, etc..
Did you learn everything you 'really needed to know' about genetics from watching the X-Men movies? Human evolution occurs over a long time scale. Mutations fall into three broad categories; beneficial, harmful and junk. Junk mutations are the most common and do nothing, as far as is currently known, save create clutter. Harmful mutations are either recessive or dominant, but the worst dominant ones typically die out in a single specimen never to enter the full gene pool. Recessive baneful mutations crop up as traits only when both parents pass a copy to children so they remain in the gene pool for a long time even when lethal. Beneficial mutations are extremely rare and to a certain extent might be considered 'case' sensitive. For example, if you live in an area with a lot of Malaria, then the gene for sickle cell provides a reproductive survival benefit as it decrease the severity of malaria.
Size increases in the last couple of centuries are a result of (to paraphrase realtors)
Nutrition, Nutrition, Nutrition
not genetics.
Your conclusion, that diverse ethnic groups will interbreed to commonality, is based on the faulty assumption that people will choose to interbreed to a great enough degree to limit differences. Humans are programed to find their own "group" most attractive - some diverge from this, and certainly the exotic beauty of some is undeniable, but on the whole, for average people, a preference for their own appearance is the norm rather than the exception.