I also find that works are arranged for this time on the belief that fewer passengers are travelling, yet we find that East Coast were having one of their busiest days and many of their trains were sold out. There is a trend to believe that commuters are the only passengers on the railway that matter.
There's a reason why East Coast prevent use of Rewards tickets at Christmas, and there's a reason why they remove peak restrictions at that time too.
The days around Christmas are extremely busy, but- as you say- the travellers are not businessmen so nobody really seems to care. There's an attitude that leisure travellers can choose when they want to travel, and at Christmas that isn't really the case.
The article was an interesting insight, but seemed to boil down to "passengers get in the way of running our railway". You see this time and time again with Network Rail. They seemed genuinely surprised when every single stakeholder objected to them closing the WCML for the whole of August 2014, for instance, as though because it's August nobody needs to go to work or wants to use the trains they've paid a fortune for. And now, when they've made a Horlicks of the Watford upgrades, the excuse is "we wanted to shut it and you wouldn't let us".
Network Rail do need to be realistic about how long work will take, rather than "massaging the figures", but as we saw with the WCML upgrade 15 years ago work will always expand to fill the time allotted. But if they'd planned to shut Kings Cross that day, with the mainline trains stopping at Peterborough and GN running frequent 12-car shuttles to Finsbury Park, all would have been well.