The issue is when to make the decision.
If it takes an hour to mobilise staff and evacuate a full train that is not in a platform, and the estimate for getting the train moving is half an hour, then it makes perfect sense to not evacuate the train.
If after half an hour, the estimate to get the train moving is still half an hour, then it still makes sense to not evacuate the train, despite the fact the passengers have now been stuck for an hour in total.
When the number of trains is many, the number of available staff is few and the estimate to get everybody moving is half an hour, then you can understand why the same decision was reached.
The reason why the estimate to get trains moving was always half an hour away was not related to ice or the original breakdown, as far as we know that was cleared in a reasonable time. It was the domino effect of multiple passengers self disembarking from multiple trains that led to the extended delay.
A lesson that must surely be learnt from this episode is that this behaviour should now be expected, which may lead to a different plan next time.
The particular circumstances of this event were very unusual, if not unique. Multiple trains stuck in a very complex area of the railway, with no power and no opportunity to bring in a rescue train. Add in terrible weather, poor visibility, third rail electrics and passengers on the tracks headed in multiple directions and you have a disaster waiting to happen. The fact that nobody has been reported injured or worse is one of the few positives to come out of this situation.
Looking at the positives:
1) Nobody was killed
2) Power was cut promptly once passengers were found on the track ensuring (1)
3) At least some of the train drivers tried to communicate as best they could with the information available
4) The emergency services responded well
5) SE twitter tried to keep passengers informed
6) All trains were moving 4 hours after the incident began
Looking at the negatives
1) The SET plan for emergency evacuations in this area was either not implemented or was ineffective
2) The SET plan for dealing with a broken down train near a complex junction needs reviewing
3) The training of platform staff to assist in an emergency needs rethinking
4) The communication between SET control and their drivers needs improving
5) The toilet facilities onboard metro trains needs fixing
6) The emergency timetable which led to the crush loading needs replanning
7) The on-train facilities in the event of a power failure need revisiting
8) The social media team need better training on how to react in a crisis
9) The ability of 3rd rail to cope with winter weather needs redesigning
My fear is that all of the above are too difficult which means that train companies will take the low risk approach and simply cancel services more quickly in the event of poor weather.