That would indeed cut quite a cost (monetary and environmental) due to barriers, tickets, validation machines, selling points, controllers...
Fun fact from history: here's a photo of former French president Jacques Chiraq jumping the metro https://cdn.radiofrance.fr/s3/cruiser-production/2019/09/904...
Ass soon as you see a human it costs a fortune.
Also around here i've never heard railway unions ask for better compensation, because as you have pointed out they already have a good pension and some transport-related privileges [0]. However, they have been relentlessly asking for better working conditions in order to provide a better service, and that the government stop to try and privatize railways.
Of course the managers, directors and private contractors earn a lot more and that's how our corrupt overlords get away with siphoning much public money into private pockets. But these people, we could well do without, and unions would certainly support a move like that.
[0] Personally, i would strongly approve better compensation for those people we now call "essential workers". It appears in a capitalist system, those who work the most and provide the most services (railway/construction/food/cleaning/health workers) are those paid the less and with the most grueling working conditions. That's a shame.
That is, without mentioning that controllers have to be paid without providing any service. If you additionally take into consideration the social cost of the criminalization of something as banal as using public transports when you don't have money, the benefits of doing away with these systems of control arguably are even more evident.