That would be unfair for non-native English speakers, who'll have to learn one more language than native English speakers.

Multiple languages has always meant someone is going to be screwed over in some sense. (Pity the Papua New Guinean whose language is not spoken even the next valley over, much less worldwide...) The main thing is to minimize the damage done.

see http://languagelog.ldc.upenn.edu/nll/?p=3175 -- though those effects require that you actually speak both languages on a daily base for years.

That's interesting, but your caveat aside it's not all that great: one has a less than 50% chance of Alzheimer's so the expected time-value of 5 years of protection is 2.5 years, at the least valuable time of your life, in exchange for let's say a year of your youth? Not a slamdunk case for bilingualism (much less tri or quadlingualism, which is just plain signaling).