None of which gets us away from the basic cost consideration of building, leasing and maintaining lots of extra coaches to satisfy the people whose fervent belief appears to be that if it's an express train the formation must be miles long all the time, no matter where it is operating or the time of day. We could go on about 'x, y or z might happen' all day long - the old GWR Class 800 thread was a classic in that regard. Plymouth, a station purpose-built for splitting and joining trains, supposedly couldn't cope with doing just that.
Does anyone seriously thinks that traffic at Lincoln is ever going to be on a scale that would justify laying on 620-seat trains to and from London every couple of hours all day? Hull Trains has been operating for almost 20 years, has a rather bigger key target market and is getting new five-car trains. I think that's what's called a bit of a clue as to the cost factors at work here when it comes to things like serving destinations beyond the core on LNER, GWR, or anywhere else.
If all big trains, all the time, was the approach adopted on GWR, that would require the provision of another 232 coaches - which would be getting on for 20,000 more seats. Even if the Tories and Labour both seem to have found the magic money tree forest somewhere, no one is ever going to pay for that kind of capacity overkill.
Nor is it going to allow an operator any flexibility to serve places that can fill plenty of seats on a limited London service each day, or try out a new route or type of service, at a low cost, and see what happens, if it is stuck with the costs of using a 600+ seat train from the word go.