Bonin would have had to realize that the plane is in alternate mode in the first place in order to react correctly. It probably never crossed his mind since he never trained in it, and conversely if he had trained in it, he may have been more likely to at least check.

But to me the bigger problem seems to be that such an important change in the plane's behaviour could happen without anyone noticing. I'd consider the mode to be something the pilot must be made aware of, not something he has to deduce from the fact that the airspeed isn't available.

Perhaps the mode is shown prominently and the pilots just didn't notice it in their state of panic. Making it more prominent probably leads right into an insane arms race - the stall warning was as prominent as anything can be and still got ignored.

I don't envy the person who has to design a airliner cockpit's user interface and decide which of a hundred potentially vital pieces of information should be displayed how.