Could Marylebone not also be expanded? What about Euston being used on occasions too? I'm not a railway buff, I don't know how feasible this is. I do know that the British travelling public deserve better treatment. Just ruining people's weekends so commuters aren't affected is not a proper business plan.
The issue with using other routes is the lack of train crew knowledge these days. Maylebone, Euston, St Pancras, Kings Cross all have different Train Operating Companies(TOCs) running into them. So Drivers and train crew will have no knowledge of travelling into those stations. In BR days, Pilot drivers were provided to guide drivers off other areas. Today, TOCs barely have enough drivers to run their own services without having to provide additional driver to Pilot trains of other TOCs. Then there is the issue of pathing into a workable timetable, which is no longer a simple thing to do as the skill base has gone.
A lot of older railway staff, or the days of British Rail, work in frustration at how the passenger is the last thought of by the TOCs. TOCs are only interested in their trains running on time, and forget that those trains are run for passengers, without whom, there would not be a railway. The dedication of railway staff is disappearing fast as we get new staff come in. Some show the dedication, but increasingly more and more now only see it as a means to make money than a vocation and being the best they can and provide a public service. It is not just the railway, it is in all industries. Society is different, people expect different hings, attitudes change.
Frontline railway staff, drivers Signallers and platform staff have their hands tied. Whereas frontline staff would make instant real time decisions for their experience, it is now all in the hands of loads of Controllers, who cannot make instant real time decisions because of the lack of experience, not being on the front line and have no concept of what is happening on the ground. Decisions are bounced around TOC Controllers, Network Rail Controllers. Telephone conferrences are held, but the Front line staff aren't involved in this. And Controllers make the final decisions. Meanwhile 30 minutes, and hour later, passenger number have now doubled, frontline staff have no idea what is happening and cannot inform passengers what is happening as they do not know being left out of the loop. It is so slow and disjointed because of the chain of people/companies involved in getting anything done. TOCs rule the roost, it is their trains, their shareholders that gain whilst their customer, the passenger suffers.