There is no realistic means of knowing what either BA or AA will be operating next year and thus not worth being overly concerned just now. I would book the BA flight because it at least will give you a rebooking option if it is cancelled or the schedule changed.
At the present time, AA still protects transfers on separate tickets from other OW carriers, but there appear to be cracks in that policy and it is far too easy to speculate whether the policy survives the current onslaught. Once things steady, you can then evaluate onwards travel to MEM.