If a building is not listed there is literally nothing the council can do to prevent it being demolished, the owners don't even need planning permission in many circumstances. As regards your specific examples
1. No decision has been made on the Gaiety yet
2. "Splott University" was not listed, and the Welsh Government refused requests to do so
3. The CRI has not been demolished, in case you hadn't noticed
4. The York Hotel had been a ruin for at least as long as I've lived in Cardiff. Also, not listed.
In any case, the real threat to our architectural heritage is not the loss of Victorian buildings, which everybody now loves, it's the loss of buildings which are 50-60 years old: old enough to be unfashionable but not old enough to be "classics" worth preserving.
This is why we lost so much Victorian/Edwardian architecture after the war, and it's what puts 50s/60s buildings, particularly in currently unpopular styles such as Brutalist, under threat now.
(This excludes Golate House, which is appalling and should be demolished immediately

)