I have several concerns about compulsory reservations in general. I have never travelled on LNER, but can't see it being practical on many other routes were the concept to spread.
1. XC services, and routes such as GWRs Cardiff to Portsmouth which see a lot of smaller journeys made on various sections of the route as well as much longer distance journeys using all or most of it. The trains are simply too short to have a meaningful mix of reserved and non-reserved coaches. Requiring a reservation between Cardiff and Newport, or Bristol and Bath is pointless. I used to regularly travel between Bristol and Southampton, and sometimes it would be busy, I often had to stand, sometimes as far as Westbury. I simply wouldn't be allowed on such a service at short notice. (There was no viable coach route at the time, it involved two changes and took half a day, I don't know if that's changed. I didn’t drive then, so no real alternative.) There isn't a clear-cut Express vs Commuter/Stopper set up in much of the UK. Much service coverage is provided as an alternating mixture of express and 'local' services, and as said before, some express services cater to both types of customer.
2. Disruption can quite often last all day. Some people have suggested reservations would have to be suspended on the next service to allow people to travel should their train be cancelled/connection missed, but rarely is disruption limited to just one operator or service, or hour in a day. This would upset people because:
3. If we are to drastically reduce capacity at peak times/holidays/sporting events etc (I can't imagine longer trains being affordable with the loss of capacity and ticket sales) then the resulting increase in cost per ticket to run what is undeniably an expensive railway to run will come with the benefit of a guaranteed seat - until it doesn't because the reservations have been suspended. When the cost rockets and the actual benefit ceases for hours or days at a time, people will be in the same overcrowded situation but will have paid a higher price. If their service runs on time, no delay repay. Will you be able to claim vouchers from customer services because the train was rammed? I can't see that being sustainable. You can't just "put another train on" as you can with a bus/coach without a lot of planning (and adding extra coaches/carriages to a train is usually robbing Peter to pay Paul, a big problem for reservation systems too I imagine!)
The industry relies on its income from high numbers of passengers. Limiting the revenue by limiting the number of people who can access the service is not the way to reduce overcrowding - it can't be economically sustainable in the long term. There will be a reduction in services, facilities and staffing.
4. Advance ticket pricing makes the railway affordable for some people (and businesses) by helping fill seats on less busy services but the restriction to that single service causes so many problems, particularly when they are told the cost of the walk-up fare when they miss it. Ask any ticket office clerk, gateline assistant, train manager/conductor who deals with it daily. With that in mind, a fare pricing overhaul purely based on availability means you'll never know what a ticket should cost. Some airline websites bump up the cost of fares if you visit multiple times to make you panic buy sooner rather than later. Is this the direction we want to go in? How about a multitude of petty fees to cover the reduction in tickets available to sell? Baggage, window seats, tables, wi-fi, charging points... I'd be cautious about airline models concerning pricing. Can you imagine a guard charging two different customers different excess fares for the same ticket/journey, because the price has now gone up in the interim?
5. Enforcement. Will the National Conditions of Travel be changed? If only "long distance" requires compulsory reservations then how would this be defined, and enforced?
6. The concept of booking a new reservation 10 mins before a journey is a great idea until half a rugby stadium tries to do it at once and the app crashes.
7. People won't "choose another weekend to travel" - its not how the world works. Stag/hen doos, birthdays, sporting and music events, religious festivals and public holidays, even organised protests happen on dates set by other people (even if more people work from home, for the majority the weekend is at the same time). They want to maximise their free time. If they can't travel/arrive within an hour or two of a time that works for them they will choose another way, and they won't bother checking the train next time.
8. Season tickets and rovers, plus payment by warrant - I don't see how any of this is possible if reservations are compulsory and there are no reservations available on the day, or reservations can only be made at the time/point of purchase.
By reducing capacity, raising prices and potentially leaving people stranded if they fear not being able to re-book at the last minute, we will kill rail travel in the UK. It'll be an expensive, unsustainable and short-lived club.