Let us not labour the pint - but there is a very decent bus service from Carmarthen to Aberystwyth - there is apparently a Cardiff to Aber bus which replicates the good train servie from Cadiff to Carmarthen.
Monday-Saturday, there are hourly buses between Carmarthen and Aberystwyth. I've still not done a long journey on it (the full journey is normally 2hrs 15mins or more), but I've done short hops and was far from impressed with the quality of the buses used. The Cardiff to Aberystwyth service is a coach, two trips Mon-Sat and one on Sundays, not been on that so I can't say if it is any good. The Carmarthen-Aberystwyth bus is only four trips on a Sunday.
Overall, I wouldn't describe the current Camarthen-Aberystwyth links as a 'very decent' bus service at all. Adequate for intermediate journeys perhaps, apart from the poor Sunday frequency, but for long-distance journeys there's a lot of room for improvement.
I think the focus should be on reopening Caernarfon to Bangor, with the potential of extending to Afonwen and rejoinin the Cambrian. This scheme seems to be slightly more realistic.
The TrawsLinkCymru group promoting Carmarthen-Aberystwyth also have asperations for Bangor-Afonwen and speeding up Carmarthen-Cardiff I believe, but Carmarthen-Aberystwyth is their 'phase 1'. Personally, I think they should have put Carmarthen-Cardiff improvements plus Bangor-Caernarfon at the top of their list, along with safeguarding of alignments for Carmarthen-Aberystwyth and Caernarfon-Porthmadog*, in the hope that doing Carmarthen-Cardiff and Bangor-Caernarfon would prove successful and help make the case for more. Starting with Carmarthen-Aberystwyth, probably the hardest of the three, doesn't make sense to me.
* that's another point I differ with their opinion, rather than using the old Afonwen route throughout I like the suggestion I've read elsewhere of trying to find a new route from Bryncir to Portmadog to cut the corner.