I've worked Boxing day and several New years when I worked for the railways. I did have the chance to work Xmas day once. There are staff about working, but mainly involved in engineering work. There is a surprising willingness among *some* staff to work, it just depends on the person as always. Some people, believe it or not, hate the idea of spending Christmas with their family, others wouldn't miss it for the world. There are 25% of households without cars, and many more depend on public transport where there are cars. There's also a growing percentage of the population that don't celebrate Christmas.
From the railway's point of view the main disadvantage is missing out on that window to do those really big engineering jobs where you need to shut the lot. That said, if NRs 7-day railway comes about with new ways of track and signalling installation being proposed, who knows.
In conclusion, if they can solve the engineering issues, then I see no reason not to have a basic service on main lines and around large conurbations.