In the days of British Rail, if there was reason to close a major piece of railway – which itself happened a lot less often than it does now – then British Rail would find a way to run trains on a different route.
For example, in October 1991 BR closed all lines through Watford Junction. What did they do to the trains on the West Coast Main Line running between London and Manchester, Liverpool and Glasgow? They moved the trains from Euston to St Pancras, used the electric locomotive as far as Bedford, diesel-hauled them over to Nuneaton, and had them go on their merry way to Manchester, Liverpool or Glasgow. Result: journey times a little longer, plus a slight walk from Euston to St Pancras, but the trains still ran.
Nowadays, what would probably happen is that Virgin Trains would throw their arms in the air and moan loudly, before getting huge fleets of coaches to bus people between London and Milton Keynes. Result: even longer journey times, forcing people to use coaches (which many don't like), and forcing them to change at Milton Keynes, all of which probably reduces the number of people travelling.
We have lost the subtle science and exact art of diverting trains.
If Virgin Trains were to try and divert their trains from Nuneaton to St Pancras via Bedford now, they would have to pay Network Rail for running trains on lines they don't have rights to do so. They would probably also have to pay East Midlands Trains, because they're very likely to disrupt their existing services out of St Pancras.
Of course, if there's reason to divert the trains in the first place, there's probably engineering work going on, in which case Network Rail are probably paying compensation to Virgin Trains. Whether or not the compensation is less than the track access charges would be, I don't know.
There doesn't seem to be anyone trying to ensure that the passengers get the best service that's possible, even when there's maintenance going on. All the train companies are concerned with is the bottom line, and Network Rail is far too busy actually maintaining the track, which is a huge job. And all the Department for Transport seems to care about is getting as much money as possible out of farepaying passengers.
That's what it comes down to: money.