You're currently fortunate enough to enjoy those terms and conditions and no-one can blame you for taking advantage of them, it's not the fault of individual employees however the world has moved on in the decades since BR▸ and most people get used to variations in their working practices (even the Civil Service!).
Nearly! As a former Civil Servant of many years' standing (plus quite a few sitting down) I was never required to work weekends or evenings. Over the years, I took on a number of job roles that could not be done during office hours alone, and worked many weekends and other unsocial hours. I was paid a decent premium, the eventual withdrawal of which fortunately coincided with me deciding that I had enough money and preferred free time instead.
As the world moved towards a 24/7 model, so did my former department. New staff for contact centres were recruited with terms of employment that included variable shift patterns including some weekends. They didn't mind, as it was exactly what they had applied for.
Then, a couple of years back, that same department launched a campaign to get everyone on the same terms, including those who like me weren't employed as the office equivalent of battery chickens. After much talking to the unions, a deal was hammered out, which made adoption of the new contract purely voluntary, but with a bigger pay rise that year for all who agreed to it. I signed up, and my new terms and conditions came into force on the 1st of the following month.
The point is that even the Civil Service saw the benefit of persuasion as opposed to threats and coercion. New railway staff are needed, and at first sight it would seem to make sense to hire them on terms that are more favourable to the company, and maybe offer inducements to existing employees to switch. Retirements and resignations will soon whittle down the numbers on "legacy" contracts. The problem, though, is that whereas my old department will continue in some form indefinitely, First might find themselves handing their staff over to another company before any negotiations have really got under way. That process would be greatly complicated by having staff employed on a variety of different terms. To make matters even more tricky, many of them have skills of a very portable kind, in considerable demand by other companies. My thinking is that First can't completely solve the problem alone, but that no-one else is going to help them do it.
How did I get on with my new conditions? Having put it off until after the pay rise, I retired 10 days later. I can see similar being a danger for
GWR▸ too.