(It is strange though that the main argument given on this forum for scrapping the majority of north-south has quite simply been "nobody wants to travel from Holyhead to Cardiff".)
For me its more about the markets on the Wales & Borders routes that are unserved (or very badly served).
The Marches used to have regular services to Bristol (I'd guess that there's some demand to get from Shrewsbury/ Hereford to Bristol)
Cardiff used to have daily services to Liverpool - I'd imagine that there'd be a reasonable "end to end" flow on this route.
There's no services from Liverpool along the North Wales Coast, despite the trade between the places.
Whilst Wrexham is a big place in potential commuting distance of Manchester and Liverpool there's no direct services (apart from a
23:01 service from Wrexham to Manchester?).
Crewe used to have direct services to Llandudno (people were discussing here recently how Staffordshire people used to travel there in decent numbers).
The North Wales Coast has lost its fast services to/from Birmingham (with them all running the slower route via Shrewsbury and Wrexham rather than the fast way via Stafford and Crewe).
There's other markets out there (e.g. Aberystwyth misses out through poor connections at Shrewsbury).
Instead, we've seen the Holyhead - Cardiff services increased from the previous one-a-day to bi-hourly plus the subsidised daily flights from Anglesey plus the "Premium Class" service, with a strong suggestion that the trains will be hourly before long. Seems to be a lot of attention focussed on one route to the detriment of a number of others.
Some unserved markets aren't Arriva's fault, of course - the way that the Wales & Borders franchise was carved up means that some routes that British Rail used to provide would now be in another TOC's territories (e.g. Swansea to Birmingham, Swansea to Bristol, North Wales Coast to Yorkshire) - such is the "private" railway.