Unless they are calling for grounding the NG fleet as well, this sounds like arm twisting American Airlines to give ground on the contract negotiation the APA has been working on to get a 20% pay increase and retroactive pay.
The leverage for twisting is that these two models will require training that no other aircraft requires, but the airline has expressed they need to train more pilots currently to stay competitive. So, if there is no extension then American Airlines might not get planes they need for particular routes AND when they do get them, there might not be training available for a significant time.
And what of the pilot of an NG who expects one sort of alert and gets a different one? Or a long term NG pilot moved up who doesn't get the sort of alert he expected.
I agree with FDR - the best plan is to let the 737 series retain the interface it has now and expect that this is the necessary end, though I am unsure that alerts for critical systems necessarily improve safety. What I am sure about is inconsistency can generate a far greater safety issue as the initial MCAS software did.