A generation ago in 1966 the railway from London to Manchester and Liverpool changed over to new, much faster, more frequent electric trains. It cut the BEA passengers to Manchester in half and contributed to the collapse of British Eagle who had the Liverpool run. But it won't happen again.
One of the structural changes to domestic travel is the percentage of passengers who are ultimately travelling to/from the city centre (and therefore more favourable to rail) continues to fall, as both homes and jobs move out from city centres. And cities expand most strongly, and have the greatest proportion of new high-income jobs created, in the direction of airports (eg London/Thames Valley; South Manchester).
Many business travellers start their day direct from home, and find getting to the city centre station more inconvenient than the airport. Increasingly they live outside the city, not inside it. And with the amazing decision of Virging to give 50% of the new trains to First Class (4 coaches of each class), if you get on the train at it's suburban Watford stop you can all find that all the standard class seats are full !
There is also the way in which, as commented, the railways have screwed their yield management policy. The 1966 change was accompanied with cheap, turn-up and go tickets, which passengers loved. But Virgin are wedded to either grossly restrictive tickets tied to individual reservations that can't be changed, or high fares. I don't use them on business to the North West any longer. Takes longer, costs more, liability to delay is greater. Plus incidental things like station parking not as well done as airports. VLM from London City is often half the price of the train.