"A train company not employing enough staff is because it's run for the benefit of the train crew."
*facepalms*
Many train crew actually look forward to their days off just like workers in other industries. Many train crew have families or other commitments, yes, even volunteering for charities in their free time, which prevent them working those days off even if they wanted to.
So, with some exceptions (there's always the "money men" in any job) most would be quite happy with more staff employed if needed.
However, staffing is worked out on something like 2.2 people employed per weekly diagrammed rostered turn, to cover for sickness, training and leave.
Where this falls apart is when people leave at short notice to join other companies (9 months' training but only 4 weeks' notice required at many train companies). Or when a badly thought out roster causes high fatigue and thus high incidents, with crew then off duty under investigation. Or when a pandemic arrives and halts all training for many months (and remember that in this case, Wales has greater restrictions still, too). Or when new trains arrive and all drivers need training handling on them, but there are hardly any instructors left. Etc.
All of which can and should be communicated by TfW management to the public in a concise and simple way, although one does wonder whether it would be simply dismissed as "train crew fault" anyway...