People commuting habits have changed in the last few years, especially into Cardiff. Its very noticeable how the morning and evening rush is not what it used to be. The only time Cardiff Central is really busy these days is when a major event is taking place. If you look at CDF or CDQ exit and entry data, it still has a long way to recover ~80%, But Places like Penarth, Aberdare, Potypridd and Rhymney are still way down, only 50% to 66% of theirs peaks in 2017/18
The disruption caused by the metro enabling works hasn't helped and could not have come at a worse time, I wonder how difficult it will be to get some people back on board once most of the metro work have finally finished.
If you close a line for 6 weeks or 6 months then people get used to finding other ways to commute.
I know at least two people who stopped using the train due to constant cancellations and disruptions, they risked their employment due to being late so often. One changed jobs (WFH) and the other bought a car to commute.
TfW will have a hard sell to get people like this back.
(Sorry this post should have really gone in the Cardiff Metro Section)