the other angle to this specific case is that there isn't a huge pool of unemployed train drivers who could do the job tomorrow.
railways aren't quite my patch, but i understand that the process from 'coming off the street' to being a fully fledged train driver working on your own is a matter of months, not weeks.
it's also one of the few jobs where the privatisation / fragmentation of the industry has gone to the workers' favour, as some train companies have tried to cut back on training get by with poaching existing drivers from other train companies (and some wind down training towards the end of a franchise giving the new incumbent a problem that takes time to solve) and towards the end of BR days there was a demographic problem waiting to happen as they were taking on less new drivers as the freight / parcels side of things wound down.
in pure market terms, there used to be (broadly) one monopoly purchaser for train drivers' labour (or two if you count the london underground) - now there are a more purchasers out there seeking a relatively rare commodity.
in a heck of a lot of jobs, management really could replace significant chunks of the staff within days...
and in fact it used to take longer to be trained as a driver. people from my dad‘s generation, starting as guards. firemen - well when steam was still a thing. The usual downward pressure on wages and increased knowledge and skills required as different rolling stock electronic systems were brought in. fortunately Aslef it’s a pretty strong union, at least at the branch my dad was a member of. Albeit over the years, there were tensions with drivers in some areas voting for short-term benefit in some different areas of the Country. The old divide and rule.