I understood that the increased frequency was to be at peak time only. This seemed like a good idea. If it was a success, then increased frequency at off-peak periods could then be investigated.
It's only a couple of years since 2 carriage Darts were introduced off-peak. It's a while since I've seen one of these, and 6 or 8 carriage Darts off-peak does seem to be quite wasteful. My recollection is that passenger traffic growth is only in single digits, probably not enough to warrant longer trains or increased frequency off-peak.