I think (hope) that the political will only exists on the fringes of Welsh politics, and then largely only when there's an election on!
For most people in North Wales travelling to most places in South Wales (and vice versa) meandering via Afon Wen, the Cambrian Coast, reversal at Machynlleth, reversal again at Aberystwyth and a scenic tour of Ceredigion will never match the eastern route.
Of course, Holyhead to Swansea and Cardiff is much shorter by road for those who so desire.
Albeit unscientific, on my last few visits to Wales (both North and South), I have never met anybody who is doing a rail journey between Holyhead and Cardiff/points west via Chester and Craven Arms, or vice versa. I have met people in North Wales who are travelling to Liverpool, Manchester, Birmingham, and London. For Aberystwyth, I have met people who are travelling to the West Midlands, the North West, and London. For South Wales, I have met people who are travelling to the West Midlands, Somerset, and London.
Perhaps the Cardiff - Holyhead could be broken at Chester (with the Cardiff - Chester section now running to Liverpool via Halton Curve every 60 minutes, which would provide a combined frequency of every 30 minutes with the Llandudno - Liverpool via Halton Curve), with the Chester - Holyhead section starting from Crewe (taking over the shuttle) every 60 minutes?