Another complication is that AA awards lifetime status when you hit certain lifetime mileage levels. I think at 1 million lifetime miles (any source) you get lifetime Gold (oneworld Ruby), and at 2 million it's lifetime Platinum (oneworld Sapphire).

So if you're in your peak flying years and expect to wind down, but still want to enjoy some elite benefits, then, depending on how much you fly, it might make sense to stick to AAdvantage in order to reach those lifetime tiers.