People want to be near where their trains will arrive, not a multiple-minute walk across a concourse, through ticket barriers, through a narrow subway, up the stairs then a fair distance along a platform. This is particularly the case at stations with a lot of passengers changing trains and a lot of through trains with limited dwell time.
If you're changing trains and have got a long wait, chances are you won't know what platform your next train will use anyway. A central large waiting room (
inside the ticket barriers - I agree you don't want to make people pass through them any more than necessary - Birmingham New Street would be so much better if there wasn't the hastle of barriers if you go up the wrong set of stairs from the platform), with ample toilet facilities adjacent, would be useful in such cases. Then, a few minutes before your train is due, you head up to the platform (hopefully there won't be any last-minute platform alterations from one group of patforms to another, but in that case you wouldn't be any better off if you had waited on the platform anyway). By only having one or two trainloads of passengers needing to wait on the same platform together, and probably not for very long, the platform waiting rooms don't need to be massive (but would benefit from interior alternations to make them as large as possible without damaging GWR walls)*.
Last time I used them, the toilets on the platforms at Cardiff Central were awful; far too cramped and fairly poor quality. Is there really enough room on the platforms to provide decent toilet facilities? I'm not convinced, so am disapointed there aren't large numbers of new toilets proposed in the new buildings proposed behind platforms 0 and 8. As with waiting rooms, these want to be inside the barriers (I would rather not have barriers at all and ensure full check tickets on-board the trains instead, but accept that probably isn't going to be practical on Metro services and the Metro is too closely integrated with mainline services in Cardiff to make seperate Metro platforms (with barriers) possible) and as close to the entrance to the subways as possible. Given the distance of platform 0 from the rest of the station, I think two lots of toilets plus large waiting rooms are needed; one at concourse level in what TfW appear to be calling the 'West Wing' or 'Fish Jetty' (currently M&S I think, though I've never been in there) and the other (again at concourse level) in the new building behind platform 8.
* note that there doesn't appear to be any waiting rooms on platform 8 in the new plans
I also think you're overstating the value of the end walls of the platform buildings. Equipment cases, decaying phone box remains, and battered doors don't outweigh more welcoming customer facilities IMO.
I didn't say anything about phone boxes and equipment cases. Most of them are presumably 1990s (or more-recent) additions and if those are non-functional and can be (re)moved then go ahead and do that - but I assume they are functional or they wouldn't be there. The end walls of the platform buildings themselves are perhaps, on their own, not of any great significance. However, what whoever drew up these plans appears to have overlooked is the reason that much of Cardiff Central is listed;
completeness. As a whole, the station is worth more than the sum of it's parts - there's a historic character to it. It's not quite Birmingham Moor Street, but it's good. Put a modern glass wall in there as that render does and the spell is broken; from the angles you can see the glass wall, that historic character would be gone. It just feels wrong.
As I said, I've nothing against re-arranging the relatively recent interiors - for example this image from the 3D tour:
The phone box presumably works (since there is wayfinding signage highlighting it) but it's not in keeping with the station so if it doesn't work get rid of it. As for the rest, you could move the ladies' toilet somewhere else, replace the three panes of frosted glass (one in the toilet door and two above it) with clear glass and put a large waiting room inside the door and I would have no complaints. I don't know whether the style of door is original, but if not you might be able to tweak the design of it to make the window a bit bigger while still being in-keeping.
