I very seriously doubt that. Even a few FAMs with guns would not have had much chance to completely neutralise a group of hijackers, though possibly the outcome would have been different in the sense that they hadn't reached their intended target but still could have crashed the plane. About the only way to eliminate the risk would have been to shoot down anyone who behaves suspiciously, which would probably result in 99% false positives. And of course anyone with these plans is aware that there are armed FAMs aboard and they would also arm themselves in some way to overcome that problem.

IMHO the only good security measure that came from 9/11 is protecting the access to the cockpit. By reinforcing the doors it becomes much harder to repeat anything like what happened. As well scanning of hold luggage (which was mainly not done in the US AFAIK) provides some security.

The problem with the FAMs is that it they will only prevent some types of attacks. Smart terrorist will simply find another way to cause terror, likely nothing to do with any previous plot at all. Most security measures seem to protect us from the last plot, while what is needed is targeted intelligence. [Added later] And that is exactly what you could use all those FAMs for!