OK I'll give you the point for the 313s.
*However*, as someone who travels from further out and has no requirement to use Moorgate they are basically irrelevant to me, along with many others.
Problems with third-rail icing at Drayton Park won't directly affect me today, but a problem on the Brighton line most certainly would in the future.
A tiny part of the 3rd rail section on GN is outdoors, so I am not sure that bad weather will affect it that much, compared to everything south of Farringdon/City Thameslink (well Blackfriars, as again that's mostly covered).
It will be interesting to see how the 700s cope with bad weather, as coping was definitely part of the specification.