Essentially, despite what the timetable says, these days trains after about 1900 on Christmas Eve must be treated as 'optional', sadly. The TOCs are driven by contractual penalties, not service pride, and thus the cheapest option for them will be taken - cancelling trains because of a longer-term staffing issue which itself has been driven by cost-cutting - even more so that they know that their tenure is going to end soon anyway.
I'm afraid I already treat timetables as optional these days. I know roughly what time I
should be leaving, but 27 delay repay claims in the last 6 months suggest it's a bit of a gamble if you'll get to where you expected to be on time. Incidentally, I've used trains about once a week this year, usually out on one day, back two days later, so that's pretty much every time I've used the train!
But the business is being prevented from offering better incentives to its own staff by the DfT, as has been explained many, many times before. So just condemning it as “a low grade poor quality business” doesn’t really shine a light on matters, does it?
It isn’t the train staff who are making the decisions that have led to the cancelled trains, which are due to insufficient numbers of said staff existing in the first place.
It's not this staff making those decisions, and credit where credit's due, the government seem to be able to make a right dog's breakfast of any thing they touch. However, there are numerous examples of staff who seem to agree with DfT that things would be much easier and simpler if these passengers would just go away, which doesn't create a great impression, as seen in the post you were replying to.
Because it should be treated as a system? There are 41.7 million licenced vehicles in the UK. How much modal shift are you expecting before the railway couldn't cope?
At busy times? I'm not sure it can cope now, never mind if a couple of hundred extra passengers turned up in rush hour. There's probably some leeway outside of that, but even when I try and get off peak Avanti trains, they're still pretty full. Aside from that, it feels like one little incident can knock the railway out of kilter over a huge area - a track defect in the Trent Valley can make trains running between Penrith and Glasgow late, but a pothole in Rugby doesn't affect those in Carlisle, for example.