Clearly they are worried about people piling over the platform edge, hence
the barrier, so why didn't the gap in line with the steps and the end of the
bay (all of about 20ft?) get filled in, so that pax from and to the bridge
would move along the platform, in stead of across it?

Because trains do not go further the platform - this platform is very
long and the first door of the first carriage is usually right
opposite the new bridge entrance.