I fail to see why someone moving to North America should seek status in AC unless they are actually going to be flying a lot of AC. If you're not going to be flying on the program you accrue with then it doesn't really matter where the points accrue.

The down-side of accruing on CO/UA/US is that the Biz fares don't get nearly as good bonus multipliers for the premium cabins as the European and Asian programs do. But you'd get upgrades and such when you're flying domestically once you do move.

Win some, lose some.