This goes back as far as approx 1990 when Cardiff Valleys decided not to run a service but this was when people's habits and shopping patterns were different. Since then, it has been carried forward through the franchises and ATW wouldn't do anything about it because it wasn't a franchise commitment, and even if they did try, they would have lost money on running a service. I have read, when this topic has been raised in the past, that the Valley guards are not contracted through the franchise agreement, to work on New Years Day. It is a farce Grangetown, that Ebbw Vale has a train service but Rhymney just over the hill doesn't. Grangetown, Eastbrook, Dinas Powys, Cogan, Barry Dock and Barry Town have a part service because the route is also served by the VOG service.
Fingers crossed for 2020. It is an issue that has nothing to do with the Metro.