couldn't AA just go through their records to calculate your BIS miles? or would that take too much effort? I suppose AA could just honor all miles up to a date as BIS miles and count BIS miles from that date on. I think AA likes their setup right now. With all miles counting, partners are buying up AA miles like crazy and handing them out to customers. AA makes a heap of money from partner miles, and since all miles count towards lifetime status, ppl are more aggressive on earning AA miles (i know I am), and partners are constantly calling up AA to buy more miles.

partner buys miles == Revenue to AA.

lifetime from all miles is a nice acknowledgement of revenue source. If you've earned 1MM, you've brought 1MM worth of revenue, whether it was from direct ticket purchase or indirect by pushing a 3rd party to do it...