The facts suggest otherwise. The Cardiff bay branch was used by 1,302,676 passengers in 2017-18. It's a fair bet that the vast majority of them were people changing trains at Queen Street of whom a large percentage would have been valley line passengers.
I do agree with you though that developments at Central Square and Central Quay will increase demand on the Queen Street > Cardiff Central route There'll be 10 tph in both directions linking the two stations though, more than enough capacity one would have thought.
I would disagree on the interpretation of the stats. Cardiff Bay may have had 1.3m passenger movements, but if we add up the Valley Lines stations which had movements of >0.4m (400,000) per year
Aberdare – 0.57m
Merthyr T - 0.51m
Treherbert – 0.48m
Radyr – 0.54m
Llandaff - 0.48m
Pengam – 0.46m
Barry – 0.53m
Barry Island – 0.75m
Heath HL - 0.44m
Caerphilly – 0.77m
Trefforest – 0.75m
Pontypridd – 0.86m
Penarth – 0.63m
That gives a total of 7.77 million. Obviously the figure for the whole network is a lot higher when you add in all the other stations (Ystrad Mynach, Porth, Abercynon and Taffs Well add another 1.4m)
Cardiff Central - 12.95m
Cardiff Queen Street - 2.9m
What is obvious is that the overwhelming majority of these passengers are alighting at Cardiff Central (and to a lesser extent Queen Street). Most to go to work in the city centre, and some are going to the Bay via Queen street, whilst some will be changing onto the main line at Cardiff Central to go elsewhere outside Cardiff.
Add in the building of new offices around Cardiff Central, and clearly diverting trains away from Central to go to the Bay is not a good idea. Cardiff Bay would be better served by improvements to the shuttle service between Cardiff Bay and Queen Street such as 2 x 2/3 car trains crossing at a passing loop or track doubling enabling trains to sync up and cross half way.