At certain airport stations, staff are known to be advised to watch out for the especially notable danger from skiing equipment bags, in case a lot of people arrive with them at once, and one or more end up overhanging the platform edge - they could easily be caught by trains, which could drag people underneath, or receive serious damage even if that doesn't happen. It goes to show that even fairly innocuous objects can pose major risks if they are not watched out for.
Other than trains actually hitting luggage, I agree the main risk is turbulence or the doors catching personal possessions, or else (probably more prominently) the turbulence of a passing train making you lose your balance.
As for alarms, positioning is very much the key in some situations. Imagine if you are on a crowded Victoria Line train and you notice an incident, but are at the opposite doorway from an alarm. By the time you've made yourself understood and had it used, things will have gone from bad to worse. But even so, people in a blind panic might not even think to use an alarm, despite the colour contrasts and prominent signage. This is of course why staff vigilance is key, and reliable detection systems & effective cameras must continue to be rolled out.
